Why did Steinbeck include the Boss as a character?

The boss deliberately put the little book in his pocket. He hooked his thumbs in his belt and squinted one eye nearly closed. "Say--what you sellin'?"
"Huh?"
"I said what stake you got in this guy? You takin' his pay away from him?"
John Steinbeck could have left the Boss out of the story altogether and skipped directly to the bunkhouse where George and Lennie meet Candy. But the short interview with the Boss serves several purposes. For one thing, it illustrates what a hard time George has in life because of having to take care of Lennie. George not only has to get jobs for himself, but he has to use his wits and persuasive powers to get jobs for Lennie. The scene also brings up the question of why these two men pal around together. As the Boss says, "Well, I never seen one guy take so much trouble for another guy." George lies about this, but later Slim will ask him more or less the same question, and George will tell him the whole story about Aunt Clara.
Steinbeck needed two main characters rather than just one, because he intended to turn his book into a play immediately. In a play most information is conveyed through dialogue, so George needed a companion to talk to. Steinbeck called his book "a playable novel" because it was written in such a way that it could easily be turned into a stage play. Steinbeck devises scenes in which exposition is conveyed through dialogue, as here with the Boss. He also introduces conflict in order to make the scene dramatic. The scene between George and the Boss is one of many illustrations of the fact that George is having an increasingly hard time handling Lennie. After they are hired, George tells Lennie:
"So you wasn't gonna say a word. You was gonna leave your big flapper shut and leave me do the talkin'. Damn near lost us the job."
And finally, the scene with the Boss illustrates the kind of abuse that downtrodden men like George and Lennie have to take in order to scratch out a bare existence as unskilled itinerant laborers. After traveling all the way to the ranch in the Salinas Valley, George and Lennie could have lost the job they needed so desperately. They had no money and they had eaten their last three cans of beans by the river the night before. This kind of humiliation helps to explain George's dream of owning his own little farm and becoming independent. The Boss embodies all the bosses who exploit unskilled, uneducated, homeless working men.