What makes the literary devices and conventions used in poetry so effective?
In poetry more than any other form of literature, sound and sense (meaning) work together to create a lasting impression (effect) on readers. Poets use literary devices and conventions to create both sound and sense in their work.
The sense of a poem is its meaning; meaning is created by words and images the poet uses as well as the syntax (word order) he uses. Unlike prose writers, poets must condense their thoughts to suit the generally shorter lines of poetry. Literary devices such as metaphor, simile, hyperbole, personification, allusion, and symbolism are useful in creating word pictures for the readers. This short line by Seamus Heaney is a perfect example:
Between my finger and my thumb
The squat pen rests; as snug as a gun.
In these few lines (few words, in fact), Heaney paints a picture of a literal pen poised in the hand of a writer as well as the power of that pen to kill, destroy, and protect. The compactness of the lines, the simile comparing a pen to a gun, and the implied symbolism all contribute to the sense (meaning) of the poem.
The element of sound in poetry is perhaps more obvious, but it cannot be overlooked as a reason why poetry is effective. The rhyme and meter, as well as such elements as alliteration and consonance, of a poem add to its effectiveness.
A poem achieves its greatest effect when it is read aloud. That is when a line that does not look particularly alliterative comes alive (as in Poe's "the silken, sad, uncertain rustling") or a particular rhythm naturally flows from a particular word (try saying "monotonous" without sounding at least a little boring) adds to the meaning of the work.
Alexander Pope, in "An Essay on Criticism" demonstrates this merging of sound and sense with lines like these:
When Ajax strives, some Rocks' vast Weight to throw,
The Line too labours, and the Words move slow....
Pope says that when the subject of the poem (Ajax) has to move heavy rocks, the words of the poem should move slowly and rather laboriously, as well. Read the lines aloud to see what he means--the second line just cannot be read quickly because the words Pope chose match the meaning he intended.
In this line, the movements of a snake are emulated by the words and sounds Pope chose:
like a wounded Snake, drags its slow length along.
This melding of sound and sense in poetry is done through the effective use of literary devices and conventions.