The symbolic value of the desert represents the lack of hope in America at the time.  When I see the desert and Dust Bowl conditions presented, I see areas that were once fertile and vibrant representing a sense of desolation and the embodiment of barrenness.  On a more symbolic level, Steinbeck's use of the desert might also be exploring the idea of the promises of capitalism for many.  The lure and assurance of wealth and material prosperity of capitalism was something that was barren and empty during the 1930s.  The spirit of "striking it rich" that had preceded this time period for so long dried up, similar to the desert.  Certainly, the physical setting for the Joads' journey is one use of the desert, but I see the concept as relating to much more about the economic and social condition of the Great Depression.