The purpose of the figurative language at the end of paragraph 4 of the selection is to —

Poverty was the cage in which we all were trapped, and our hatred of it was still the vague, undirected restlessness of the zoo-bred flamingo who knows that nature created him to fly free.

This is a metaphor for the imprisonment of poverty... an illustration of Lizabeth's inner conflict... her awareness of the situation she lives in, and the impulsivity of youth.