Relate this to the great depression and the grapes of wrath book.
The Grapes of Wrath was set in the 1930's during the Dust Bowl. The drought that caused the Dust Bowl, also caused farmers to lose their land and their homes....... this prompted a necessity to relocate and many people bought cars in order to do this. There were other types of transportation avaailable, but families who could afford any type of vehicle would have purchased one simply because they wanted to being as many of their belongings with them as they could. The people of the Depression couldn't afford to buy new, thus, they had to salvage what they were able to.
