I've come up with some ideas about poetry, over the years.
*A defense against mediocrity.
*It kicks you in the ass with its transformative power.
*One of the supreme exercises of the human intelligence.
*The conquest of imaginative freedom.
I'm assuming, in these definitions, that we know that it does all this through language. There are other defenses against mediocrity, other ways of exercising the human intelligence or conquering the freedom of the imagination, that do not use this particular medium. There are forms of literature in which the language used seems secondary, rather than poetic. There is even poetry that puts language in a secondary plain, though I tend to not think of that as poetry at all. Those literary forms might also do some of what poetry does through language.
The other objection is that a lot of poetry is mediocre, or unimaginative or unintelligent. It is hard to say that it is a defense against mediocrity, then.