Steinbeck only uses Slim to give himself the opportunity to explain to the reader why George and Lennie, who are different in so many ways, travel around together. Slim is observant but not naturally curious. He happens to be the best character for George to explain things to because he is obviously intelligent, friendly, considerate and understanding. Steinbeck wrote this novella in such a way that most of the expository information is conveyed in the form of dialogue rather than in straight prose. This was solely because Steinbeck had made arrangements to convert the book into a stage play to be produced in New York the same year the book came out, which was in 1937. The dialogue in the text could be turned into the dialogue spoken by the characters on the stage. When George is explaining some of the history of his relationship with Lennie to Slim, he is actually explaining it to the reader and to the future theater audience when the same dialogue will be spoken in the play. Throughout the novella it will be noted that Slim is just a quiet, thoughtful observer, not a busybody, not nosy, a philosopher who is willing to live and let live.
Steinbeck obviously felt the need to explain--to the reader and to the future theater audience--why it was that two men who were not gay wanted to travel around together and shared a dream of living together on their own farm. When Slim comments:
"Funny how you an' him string along together."
"What's funny about it?" George demanded defensively.
The word "funny" and the word "defensively" are both significant. George must realize that it looks a little bit "funny" for two men to have such a close relationship. No doubt George has gotten funny looks and has been asked impertinent questions about the subject in the past. If he wants to own his own farm, why doesn't he find a woman to share it with him? There must be thousands of women who would love to share his dream with him. This is what subsistence farmers have been doing for hundreds of thousands of years, since the beginning of the so-called agricultural revolution.
Steinbeck evidently felt he could not have a man and a woman as the main characters in the story he wanted to tell. A woman who traveled around on boxcars, slept in hobo jungles and in bunkhouses with a bunch of men, and did back-breaking field work would not be realistic. Even if a woman wanted to lead such a life, she would find it nearly impossible to get jobs.
An excellent novel about the simple life of subsistence farmers is Growth of the Soil by Knut Hamsun, a Norwegian who won the Nobel Prize in Literature in 1920.