The purpose is to inform.
It is not even clear what the author would be selling. Also, they're not trying to entertain or persuade; there is no indication of this; so the best answer is "to inform".
Science literature is often held back by facts, where you can express yourself, like an expository essay since they are held back by facts as well. Plain fiction is all up to the author with nothing holding them back, like an argumentative essay would be since it’s opinionated.
Answer:
Ummmmmmmm is this supposed to be a song lol