what purpose does slim serve for the author?

Slim is basically a calming force in the book. He is level headed and respected by the men. He is a confidant to George. Slim serves as a sort of rational voice in the midst of all the loneliness and eventual chaos in the book.

Overall slim is the man of the ranch, and even curley doesn't dare to threaten him. He is respected and appreciated by the fellow ranchmen.