Many poems have those features we look for in Books for Learning, such as rhythm, rhyme, repetition, and alliteration.
Poetry is also a wonderful way to introduce older students to age-appropriate but very simple text.
See Emergent Literacy Success, pp. 213-233 for more information and ideas. Poetry from the page to the stage. This great book provides ideas for teachers working with elementary education students in poetry performance activities. Lets Start Communicating.