Fitzgerald seems interested in showing that Tom Buchanan has upper-class status and an upper-class income but lower-class values and tastes, including his tastes in mistresses. Myrtle is definitely of the lower class. She is pretty but cheap and immoral, as is shown in her conversation, her associates, and the things she buys, including a one-dollar mongrel dog for ten dollars. She is married to a nice but unintelligent and uneducated man who obviously has no future. It is appropriate that they live on the premises of a dying business near a dump-yard. Tom is not democratic but just common and unintelligent. He is comfortable with people like Myrtle and her sister and their friends. For plot purposes it is appropriate that Fitzgerald should have place Myrtle in such an ugly setting because it easily explains to the reader why this woman should be so strongly motivated to get away from it. It is her urgent desire to escape from the place that leads to her death and subsequently to the death of Gatsby.
The valley of ashes is symbolic of the hopelessness that dominates the lives of its residents, including Myrtle and George Wilson. Those unfortunates who live in the valley of ashes cope with gray lives, gray houses, gray prospects for the future.
Myrtle lives above George's garage, within the valley of ashes but close to the edge of the area. The closeness to the outside world is important because it reflects the opportunities Myrtle has to escape from George and her valley home when Tom takes her away.
These events occur at Tom's convenience; Myrtle would never be able to get away from the valley by herself but could imagine herself in another world every time she threw "a regal homecoming glance around the neighborhood" and entered "one slice in a long white cake of apartment-houses."