Valid question. Two answers:

Father Weber, in his blog, gives a compelling answer (click here for short article)

Also, Cynthia Bourgeault, the author featured in this page, explains it eloquently:

The Four Senses of Scripture

Like all the Western religions, Christianity is a religion of the Word. But that Word is a unitive Word: a heart word. It does not yield itself up easily to a linear, or cause-and-effect, way of thinking. At the literal level, elements in the tradition such as the Virgin Birth or the mystical body of Christ may make no sense at all. But to the awakened unitive imagination, they become precise road maps of the path of inner transformation, increasingly verifiable as those depths of inner silence and recollection increase.

In fact, for much of early Christian tradition, this movement beyond the literal was the whole goal of the spiritual journey. To awaken meant to awaken to an increasingly subtle recognition of the hidden power contained in the language and imagery of scripture, a power capable of sustaining both spiritual and moral illumination. Monks diligently applied themselves to their lectio divina, or sacred reading of the words of scripture, confident that as their hearts were purified through prayer, moral vigilance, and ascetic practices, they would become more and more capable of understanding the sublime truth preserved like finest wine within the sturdy casks of the sacred writings.’

According to monastic tradition, this deepening understanding unfolds in four stages, commonly known as the four senses of scripture. They are the milestones on the journey to unitive understanding. While the various monastic orders diverge in their nomenclature for the two middle stages, the basic scenario is the same.
The first stage, the literal, is all about facts and linear causality. Did the Virgin Birth really happen? Does Jesus really intend me to cut off my hand or pluck out my eye if it leads me into sin? At this level, the Bible tends to be interpreted as a rule book for daily living, and there is little tolerance for ambiguity.

The literal level gradually gives way to the second phase, called the christological. At this point, we begin to see all the stories and images in the Bible as pointing directly to the Christ mystery. Jerusalem, for example, is no longer just an earthly city, it is preeminently a symbol for the church, “the bridegroom of Christ.” The Old Testament images of the suffering servant and Son of man are seen as direct foreshadowings of Christ, and Abraham’s sacrifice of Isaac in the book of Genesis is a prefiguring of Jesus on the cross.

While this process may seem forced -and if you’re not a Christian, even offensive- the how of it is actually quite interesting. We are leaving linear causality behind and beginning to see analogically in terms of coincidences, symbols, and resonances– in other words, through the eyes of poetry. Around the center point of Christ, we learn how to tap into the more subtle image-forming and symbolic capacities of an awakening heart.

At the third stage of development, called the tropological (which means having to do with growth), we leave behind the Christ mystery as the template through which all emotions must be processed and allow the images to form their own patterns and cross weavings. At this stage, the light begins to dawn that the Bible stories are holograms of the soul’s journey. They are rich portraits, in analogical language, of the stages and steps we all go through in the journey of transformation. Jonah and the whale, for example, is no longer discounted simply as a myth or folktale; we see that every new beginning involves a fleeing, a constriction, a darkness, and then being “coughed up” onto new ground. Mary and Martha, the sisters in Luke’s Gospel who invite Jesus to dine with them, are no longer two different individuals, but a parable about the Mary and Martha in each of us and how the busy, self-important egoic self must give way to the heart, which knows how to sit in rapt adoration at the foot of the master. Once we begin to hear scripture, it’s like suddenly beginning to crack the soul code.

My teacher, Thomas Keating, vividly describes this stage in his own monastic journey, when his novice master assigned him the Old Testament book of Exodus for his lenten scriptural reading. Having hoped to spend Lent with the Gospel of John, he embarked on this task with a heavy heart. But soon his excitement grew as he realized, “This book is talking about my life. Whoever wrote this book must have been my psychiatrist!” He could see how the narrative of the Israelites’ escape from slavery in Egypt and miraculous deliverance at the Red Sea was a vivid metaphor for his own conversion experience

There is still a fourth and subtle stage that is the full emergence of the unitive. At this level of understanding, we become not only sensitive interpreters of the patterns but actually cocreators. The fifth-century Desert Father John Cassian once said he knew his monks had achieved this unitive stage when they sang the psalms as if they were composing them.” If, at the preceding stage, the scriptural story is “all about me,” at this last and most sublime stage, we begin to realize that there is only one story, the great Biblical drama of salvation, and our own life is perfectly mirrored and contained within it.

While Saint Benedict may not have had terms such as archetypal unconscious and unitive awakening at his disposal back in the sixth century, his intuitions in this regard were very keen. His school for the Lord’s service was, in essence, a systematic method for the awakening of the unitive imagination. And the core of his curriculum, both qualitatively and quantitatively, was psalmody.

From “Chanting the Psalms” chapter 5 – Cynthia Bourgeault

How do we start chanting? See: Chanting the psalms – For beginners

IS THERE CHANTING beyond the PSALMS?

As we have seen repeatedly throughout this book, the words themselves are of primary importance in Christian sacred chanting. And since 150 psalms translates into more than five thousand lines of sacred poetry, there are a lot of words to deal with. Classic Christian psalmody is an art of high intelligibility requiring focused attention and a willingness to engage with the images and emotions that the psalms offer up as the working laboratory for personal transformation.

By its sheer prolixity, the tradition stands somewhat at variance with many of the great universal traditions of sacred chanting at least as they are commonly taught and practiced in the contemporary West. Psalmody is not tikr (“prayer of remembrance”) in the classic Sufi sense, in which the repetitive chanting of one of the ninety-nine names of God lifts the prayer to a level of ecstasy. Nor is it toning or mantric chant, as in some forms of Hindu and Buddhist practice, working intentionally with the vibration of sounds and pitches to produce desired effects in the inner body. While the psalms are certainly considered by the church fathers to be a primary tool for “putting the mind in the heart,” they are usually associated with the preliminary, or “purgative,” phase of the transformative process, where they lay the groundwork for purifying the passions and awakening the heart’s capacity to feel deeply about spiritual realities. The psalms in and of themselves are not classically intended to land one immediately in the contemplative state.

The Orthodox theologian Simeon the New Theologian reflects the traditional view when he assigns psalmody to the second rung of his ladder of transformation (there are four altogether), by which the monk gradually ascends to the “perfect” state of contemplation. As Simeon sees it,

Those who undertake to climb by these rungs do not begin with the top and then go down, but start from the bottom and go upwards. The method by which he who wishes to may raise himself off the earth and rise up to heaven is as follows: first, he must wrestle with his mind and tame his passions; second, he must practice psalmody, that is, pray with the lips, for, when passions are subdued, prayer quite naturally brings sweetness and enjoyment even to the tongue and is accepted by God as pleasing to him; third, he must pray mentally; fourth, he must rise to contemplation. The first is appropriate to beginners; the second to those who have already achieved some measure of success, the third to those who are drawing nigh to the last rungs of perfection, and the last to the perfect.

Both Simeon’s imagery and the teaching itself are solidly attested to within the classic spiritual traditions of both the Christian East and the Christian West. The contemplative state is to be approached only gradually, after thorough moral purification and a systematic training of the intellect and imagination.

Similarly, while one sometimes hears the Desert Fathers advocating the we a single short phrase as a form of active prayer, such as John Cassian’s famous use of the versicle from Psalm 70 “O God, come to my assistance, O Lord make haste to help me”, it would be a misreading of the tradition to interpret these phrases as mantras in the classic Eastern understanding that is, as devices to quiet the mind; to the contrary, they are capsules of pure, concentrated emotion. The reason this little phrase works so marvelously, Cassian claims, is “because it carries within it all the feelings of which human nature is capable.” Where feelings run deepest, so, too, does prayer.

To express this same idea in classic spiritual terminology, Christian sacred chanting has traditionally been a cataphatic practice. This means that it engages the faculties -reason, memory, feeling, imagination, and will- which are the foundation of our usual, or egoic, sense of selfhood. It is not intended to transcend or still the mind, plunging the participant into a direct experience of formless or unboundaried selfhood, as is characteristic of apophatic practice and a good number of the world’s sacred chanting traditions. While ecstatic moments can and do happen during the psalmody, they are not the goal of the process nor do they express its primary purpose within the traditional program of Christian contemplative transformation. The primary purpose, as we have seen, is to surface and temper the shadow material within the personal unconscious while at the same time strengthening the analogical imagination and the aptitude for metaphorical perception. Within the Christian metaphysical framework, these two “heart” capacities are seen as necessary preconditions for a unitive seeing that does not simply dissolve the particularity of the created order into a grand “Oneness,” but can instead flow out from this underlying Oneness into the infinite play of particularity.

It is important, then, when exploring the various worldwide traditions of sacred chanting, not to fall into the trap of comparing apples and oranges. Judged from the perspective of classic contemplative chanting, Christian psalmody may look unduly cerebral and rigorous. And yes, the mental operations do tend to put a damper on ecstatic experience (at least until the words and music are well in hand) and to retard the transition into direct, nondual perception. Within its own self-understanding, however, psalmody is working with a different piece of the transformational program- not contemplation per se, but the moral and emotional underpinnings through which contemplation becomes a fully integrated part of the experience of human personhood. It’s all part of that some times frustrating but always rich dialectic through which Christianity holds the tension between the unboundaried and the finite; between pure consciousness and the fragile human soul who weeps, prays, loves, and dies.

From “Chanting the Psalms” chapter 14 – Cynthia Bourgeault