I think there are as many reasons we write poems as there are reasons we eat, ranging from pleasure to filling time to survival. I have learned to beware of anything I consider easy. Writing poetry (well) is difficult because it requires compression. Every single word needs to work. I know a poem is finished, in part, when no word can be replaced by a better one. Prose, especially full-length fiction, is difficult in part because it takes longer, more self-discipline. Rarely, though, will readers of novels or even how-to manuals scrutinize a single word (unless the writer seems to be trying to show off his or her vocabulary, independent of other purposes).
Prose writers will gain from writing poems simply by practicing conciseness. Poetry writers will gain from writing prose for the mental exercise. There is no reason one shouldn’t practice both. I had a turkey sandwich for lunch one day and “duck two ways” for dinner. Both satisfied.