What is John Steinbeck's social purpose in the Grapes of Wrath?

To understand the social purpose you need to analyse the themes of the novel - that's pretty much what a theme is, to enlighten the reading audience of issues written within the book. You are correct to mention the Great Depression and how it affected families, law and punishments - the value of going against unfair laws when they interfer with a great social purpose, the ability of a family man to leave the comfort of that family and fight against a social injustice ( the near slavery conditions of the workers ). And the tenacity of families and humans to endure great hardship hoping for a social change to make their lives better.

You are also looking at the difference between human greed and the lust for power as opposed to the human values of empathy, community and family - we help each other in times of stress, we help the community by sticking together. We can be altruistic but we can also be cruel and heartless. By focusing on one point in time ( the Depression ) and one family ( via Tom Joad ) the themes are given a setting most people would recognise and empathise with.

The treatment of the workers and Tom is suppose to make us angry, to think about our social structure and the underlying fairness, or not, in life. I will recommend the SparkNotes for this book, they are very comprehensive and well written. See the source below for the link - and if you are studying the novel try watching the movie as well, it might not be identical to the book but it will help you understand the bigger themes and motifs.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.