From Glenn Arbery's "The Poetic Renewal of the World":

Whole poems are instantly present, without recourse to a text, and reciting them has its own pleasures: all that great language alive in this moment, with these friends and the very air you breathe. And on another, more personal level, these poems, once they are held for a time and revived in the memory, begin to reveal why living memory is more than a hard drive. When St. Augustine writes about memory and time, one of his major examples is a psalm he knows by heart—and by heart, with all the implications of really caring about something, is how the English language teaches us to think about what we’ve memorized.

To know something by heart means that you’ve taken it deeply into the center of who you are. When you know a poem by heart, you find that you can not only recite it, but also relive it and let it unfold in new ways, line-by-line, revealing new subtleties that you might have missed in the mere recitation. But even before this meditative exercise begins, the shaping force of a memorized poem has already laid its claim. Long before I ever make a technical point about the “turn” between the octave and the sestet in a Petrarchan sonnet, or even fully discuss what a sonnet is, those who know sonnets by heart have already felt their nature. For example, in the first eight lines of “God’s Grandeur,” Hopkins describes how a world originally charged with the grandeur of God has become inaccessible to modern man because “Generations have trod, have trod, have trod,/And all is seared with trade, bleared, smeared with toil/And wears man’s smudge and shares man’s smell. The soil/Is bare now, nor can foot feel, being shod.” Just when plodding repetition and world-weariness seem to take over, Hopkins turns the poem in the ninth line: “And yet for all this nature is never spent./There lives the dearest freshness deep down things.” Having the poem memorized means that it can be thought through at leisure; lines like “The world is charged with the grandeur of God” suddenly take on new resonance.