The main theme of the novel is Good vs. Evil, or perhaps more specifically, the never-ending battle between Good and Evil.
The Salinas Valley can be seen to represent the lands where Adam and Eve were banished after being kicked out of the Garden of Eden (East of Eden). The story's characters, like Adam and Eve and Cain and Able of the bible, are struggling with their current sins, and the sins of the past.
The specific use of description by Steinbeck is that the Gabilan Mountains to the east are beautiful and full of light, while the Santa Lucias to the west are dark, brooding, and dangerous. This sets up the Good vs. Evil theme through landscape.