Prose gets divided up into fiction and nonfiction and short fiction and long fiction and autobiographical nonfiction and so on. Poetry can do any of those things except with the added definition of intensified formal pressure.
Robert HassWhen Emily Dickinson's poems were published in the 1890s, they were a best-seller; the first book of her poems went through eleven editions of a print run of about 400. So the first print run out of Boston for a first book of poems was 400 for a country that had fifty million people in it. Now a first print run for a first book is maybe 2,000? So that's a five-time increase in the expectation of readership. Probably the audience is almost exactly the same size as it was in 1900, if you just took that one example.
Robert Hass