Steinbeck tries to show America as it was, without the romanticism of nostalgia. There are no simple people: the heroes are not without fault and the villains are not simply evil. The Great Depression and the Dust Bowl was the final blow to the terrible farm prices of the 1920s. The odd chapters from 1-11 show the history of how those who suffered came about.
America is shown as a place where there is opportunity, but the people already in place (there first) do not want those who come in (immigrants) to take what little there is left. This shows the a brief history of our immigration patterns of the settled not wanting to give into any new people coming in to "take" jobs away.