In a searching meditation, William van Ornum offers answers:
Martin Luther noted that "the entire Bible is contained in the Psalms.” The Psalms put our inchoate longings, or as St. Paul would say, groanings, into words. Wordsworth echoed this when he wrote “poetry is the spontaneous overflow of powerful feelings.” The Psalms express our feelings in hymns, pleadings, sorrows, penitence, petition and thanksgiving.
In understanding the Psalms, it is helpful to compare and contrast them to English poetry. Whereas rhyme is one hallmark of English poetry (excluding, of course, the free-rhyming poetry of recent years), parallelism is the structural component that distinguishes Hebrew poetry. While parallelism may not be as pleasing to our contemporary ears as rhyming (and this may be because of our own historical conditioning—who knows what calming and hedonic effect it had upon ancient listeners?), it served a very practical purpose in Old Testament times: since the Psalms were presented orally, the repetition of themes in a slightly different way helped create a meld of what was being expressed. The second line is often an intensification of the first, as in the beginning of the Divine Office: “O God, come to my assistance. Lord, make haste to help me.”