Unlike prose, poetry doesn't use or need full sentences. A good poem condenses meaning so as to be conveyed by a fraction of the words used in a story or novel. By choosing the most powerful words to convey all the nuance intended, poetry taps the power of words more than any other form of written communication. It takes into account connotation and denotation, they way things sound, and how it flows.
"Words are, in my not so humble opinion, our most inexhaustible source of magic, capable of both inflicting injury and remedying it." - Albus Dumbledore, Harry Potter and the Deathly Hallows, Part II