In The Great Gatsby, the eye glasses and eye doctor symbolize blindness. Blindness is virtually everywhere in the novel.
Gatsby is blinded by love to the truth of his relationship with Daisy. He thinks their love is special, poignant, earth shattering. But the relationship is an illusion--Daisy never loved him as he loves her. His past, as he remembers it, never really existed.
Wilson is blind to his wife's affair. Tom is blind to the wretchedness of his life and to the truth about the world and other people. Jordan is amoral and blind to everything around her. Daisy is blind to the harm she leaves in her wake. The characters in the novel live but don't see. Thus, the glasses and eye doctor billboard.
The desolate area you ask about is actually between the Eggs and New York City (there's only the bay between the two Eggs), and this, too, is indicative of the wasted lives and existences in the characters' lives. This Valley of Ashes is what, symbolically, is left of the characters by the end of the story: waste in a wasteland.