In John Steinbeck's The Grapes of Wrath, the effectiveness of the writing presents two important thematic motifs.
First, Steinbeck brings alive the suffering of those who needed to relocate because of the effects of the Great Depression that occurred at the end of the 1920s and continued through the 1930s. Many of the people affected were the farmers in what was called the dust bowl; these people were also known as "Okies," a derogatory term that frightened landowners named the migrant workers moving into the West. The Joad family is just one that is representative of the hundreds, perhaps thousands of families, displaced, who moved westward to find a better way of life.
Secondly, Steinbeck's writing drives home to the reader those things that kept the the migrant workers moving when things seemed so hopeless. For the Joads it was the sense of family. As the story progresses, we see that the sense of family grows to include others who are also struggling to find work in California, and survive. The theme of family and the global community are present to the very end of the novel, as connections are made with people the Joad "family" does not know, but their similar circumstances draw them together to help each other.