Poetic form as understood from the outside, that is theoretically (using the sonnet as example):
The effect of Shakespeare's sonnet differs altogether from the effect of its content when stated in prose, because the meaning of the sonnet is rooted in a host of poetic subsidiaries* which are disregarded in the prose account of the sonnet's content. The sonnet as a work of art is not merely enriched and altogether recast by its poetic subsidiaries; these subsidiaries also serve to cut the sonnet off from the person of the poet.
—Michael Polanyi and Harry Prosch, Meaning, p. 83.
* By "subsidiaries" the authors mean poetic devices: image, metaphor, rhythm, sound patterns such as rhyme, consonance, assonance, etc.
Contrast the theoretical view with this view from the inside, that is from the viewpoint of the poet:
The poem is in me, evil, alien, evil
and hateful; with scorching fire it burns my nights,
it passes through me crowdlike, hoarse with shouting
like a torchlit procession in the streets.
The poem is evil, hateful, trying to burst
its form (how hard to shackle one who's free),
and though I drag it from my fiery innards,
its master I will never wholly be.
It twists, shouting and troubled, till it cries out;
becomes then alien, a friend who never was,
stands on the frozen, flaming threshold, created,
and joins the others in the evening frosts.
—White Magic and Other Poems (translated from the Polish by Bill Johnston)
We can see how theory can describe but not embody poetic experience. Therefore my core question: Why would any poet want to begin with theory?