This is an interesting question, since Langston Hughes's "Harlem" is generally noted for its imagery, not its alliteration or rhyme.  The poem does contain those things, of course, and they do matter to the meaning.

Alliteration is all over in this work.  As the previous post reminds us, alliteration is the repetition of initial or beginning vowel sounds.  Take the very first lines

"What happens to a dream deferred?

Does it dry up...."

Note the repeated use of the letter "D."  Alliteration is generally an attention-getting device which draws the reader immediately into the piece.   It also serves to add a melodious tone to a work, which it does here.  "Syrupy sweet," for example, is effective because it enhances the image and even the sound of a smooth-running syrup. 

The rhyme is fairly simple and pronounced.   The effect of the ABAB rhyme scheme is to separate each of the images throughout the poem:  rotten meat, sagging loads, festering sores.  One after the other, the images pile up until the final explosion. 

The use of both alliteration and rhyme serve to enhance the imagery of sickness and rottenness and eventual explosion--the result of putting off (deferring) something long hoped for and longed for.