What is the purpose of citing textual evidence in a literary analysis essay?

Read the excerpt from infinite jest 'my application's not bought," i am telling them, calling into the darkness of the red cave that opens out before closed eyes. 'tam not just a boy who plays tennis. i have an intricate history. experiences and feelings. i'm complex. "read,' i say. 'i study and read. i bet i've read everything you've read. don't think i haven't. i consume libraries. i wear out spines and rom-drives. i do things like get in a taxi and say, "the library, and step on it." my instincts concerning syntax and mechanics are better than your own, i can tell, with due respect. the theme suggested by the excerpt is about a. being understood b. the value of education c. wisdom versus foolishness d. learning responsibility