Poetry is special because it presents a language that is distinctly opposed to the regular, everyday, conversational language whose only purpose is communication. Even when it is modern, free-verse poetry, it is detached from the "common" use of language because it is always highly deliberate, measured according to its inner structure, determined by the rules (or even by the lack thereof). In a speech, fiction, nonfiction in print, or play, language has a certain function - to convey a message, to achieve a purpose, to engage a reader to do something, to inform. Poetry can have any of these functions, and much more, as it always creates a separate world where every letter, word, line, stanza, or any other form has an internal artistic role within the whole.
Language is very flexible, and poetry shows this perhaps better than any other form of writing. You can write a sonnet with a mathematically firm structure and precise rhythm, or write a free-verse poem without punctuation and capital letters - it can bear any burden that you decide to lay upon it. It is the most liberal and the most constrained art form, at the same time.