Having grown up in the Salinas Valley of California, the setting of his novella, Of Mice and Men, John Steinbeck became sympathetic to the workers of this area. A socialist himself, Steinbeck rallied for those who were oppressed by writing about them and presenting a realistic--if not naturalistic--portrayal of their condition.
Reflective of the title, George and Lennie are mice in the "maze of life," inferior to the bosses who have control over them. By having them speak in the realistic dialect of their class, Steinbeck indicates this social inferiority. Having taken his protagonists from the agricultural working class of California, Steinbeck, in his social realism, presents effectively the problematic relation between the workers and the land (represented by the owners) on which they labor:
The boss said suddenly, 'Listen, Small!'....'What can you do?'
In a panic, Lennie looked at George for help. 'He can do anything you tell him,' said George.
The boss turned on George. "then why don't you let him answer? What you trying to put over?'
I often think that dialect ensures a relationship between the reader and the setting. Think about it. Each setting you are a part of requires a different vocabulary set. When you talk with your friends, you use more interesting language, with your parents and teachers, you have another set. If you were to be in an interview, you might have yet another set.
Steinbeck is a very competent writer. He doesn't use informal or inapproriate grammar structures on accident. He does it to layer the story with another characteristic that fills in the picture of their lifestyle to us. It was a completely different era that we cannot relate to. The lack of education and relationships comes through in their language. The migrant life is displayed through language.
In my opinion, the author does this in Chapter 2 (and the rest of the book as well) to make the book seem more authentic. When the characters speak in dialect, it makes the whole setting of the story more believable. It would be hard to believe that all of these men would be speaking in proper English when they are uneducated manual laborers.
I think that the use of dialect also helps to establish the relationships in this chapter. It is clear that the boss is superior to the others. Look at how he speaks -- it is much more proper than the way any of the men speak. This helps us understand who has power and authority on the ranch.