Why is the setting so important in Katherine Paterson's The Great Gilly Hopkins?

In Katherine Paterson's novel, The Great Gilly
Hopkins, I would assume the setting is important primarily because while
living in Maryland, Gilly will be required to interact with "people of color." With all
of the resentments Gilly has acquired in being bounced from one foster home to the next,
she takes her frustrations out on the weak (like William Ernest, the other foster child
in her home) and especially African-Americans. Her teacher, Miss Harris, and the
Trotter's dear friend, a blind Mr. Randolf, suffer from Gilly's lack of
tolerance.

This element of the story provides the reader
with a clear sense of Gilly's frustration with her life. It is especially obvious when
she arrives at the Trotter house. Gilly meets Maime Trotter and describes her (unkindly)
as a "bale of blubber" and a "freak." With William Ernest
(W.E.)...

She
waited until Mrs. Trotter and Miss Ellis were talking, then gave little W.E. the most
fearful face in all her repertory of scary looks, sort of a cross between Count Dracula
and Godzilla.

By the time she
gets into school, Gilly ends up making a racist card for Miss Harris, and later steals
money from blind Mr. Randolph.

However, it is only by
seeing this side of Gilly that the reader can begin to appreciate the changes Gilly goes
through when she really begins to settle in at Maime Trotter's home. She begins to read
to Mr. Randolph from the books in his library. Gilly also starts to cooperate with Miss
Harris, which enables Gilly to excel in her studies. Gilly even comes to the place where
she identifies Maime Trotter as her "mom," William as her "brother, and Mr. Randolph as
her uncle. These changes are indicative of how far Gilly has come. Without Gilly's need
to come to terms with the issue of race in her daily life at school and at home, we
would never have been able to see how far she has come by the time she is forced to
leave yet again.