What is the purpose of this poem?

The purpose is, ironically, that you can't just look at a poem and try to squeeze out a purpose or a meaning. Poetry is much more complex than that. It's not something you tie to a chair and try to torture a confession out of. It must be studied with care and intricacy. You examine it on a slide like a blood sample, or listen to it like a beehive, or drop a mouse in it like a maze. You don't force a meaning out of it. You study it, and find the meaning yourself.