The old swamper, Candy, is an essential character for Steinbeck in Of Mice and Men because he is pivotal to the plot as well as to the theme of the Fraternity of Men.
CANDY'S CHARACTER FURTHERS THE THEME OF FRATERNITY
When Candy's dog is shot, the swamper is bereft, believing himself alone and alienated in his aged and maimed condition. But, when George allows him to share in the dream of owning a ranch because he has money saved, Candy feels renewed in life and whole because of his sense of belonging again instead of worrying that he will be disposed of like his dog. For, like Crooks who longs for another man to be with him sometimes by whom he can "measure" himself, Candy finds meaning in sharing a friendship and a dream with George and Lennie, thus underscoring the importance of fraternity.
CANDY'S CHARACTER FURTHERS THE DEVELOPMENT OF PLOT
For one thing, the addition of Candy into the plot adds a third dimension to the dream of Lennie and George, thereby giving it more reality. And, this dream is the provider of hope for the lonely "bindle stiffs." Once Candy is included in the dream plans, he interacts more with the other men, demonstrating how destructive his alineation has been as it has separated him from the other men in a manner that has produced aggressiveness, hatred, and fear. But, when Candy joins Lennie and George, there is a camaraderie built among them that contrasts greatly with this separation Candy has felt before, especially once his dog is shot. Moreover, the alienation that Curley feels from the workers, a separation that generates his pugnacious and hateful behavior, is perceived as an alienation not dissimilar in nature to Candy's when it is juxtaposed with the incidents involving Candy and his dog.
In addition, Candy gives voice to many of the ideas and feelings of George and Lennie. For, when he discovers that Lennie has inadvertently killed Curley's wife, he is angry because this Eve has interfered with the men's fraternity. He looks down as the body of Curley's wife,
"You done it, di n't you? I s'pose you're glad. ever'body knowed you'd mess things up. You wasn't no good. You ain't no good now, you lousy tart."
"I should have knew," George said hopelessly. "I guess maybe way back in my head I did."
The interference of the woman has broken the spell of the fraternity of the men; Candy verbalizes this loss, and physically he illustrates it as he lies down in the hay and covers his eyes with his arm, just as he has done when Carlson shot his old dog.
Steinbeck intended to paint a word picture of a big ranch in the Salinas Valley, employing crews of itinerant workers. He couldn't introduce and describe all the men on the ranch, so he picked a few to represent all the others and to represent such farm workers in general. Out of all the men on the big ranch he really only focused on the boss, the boss's son Curley, Candy, Slim, Crooks, and Carlson, along with the newcomers George and Lennie. Introducing a selected number of farm workers to represent farm workers in general was one of Steinbeck's major narrative problems, and Candy, who was the first one to appear after the boss, is extremely useful for the purposes he serves. He is the type of character that Henry James referred to as a ficelle--someone who knows all the ropes, can provide necessary information about past and present, and holds the plot together. He sees everything, knows everything, and likes to talk. The others were mostly silent men who stayed in the background and had no dialogue. Steinbeck wrote the short novel with the intention of turning it into a stage play, and the play would undoubtedly have included a number of extras in overalls to create the desired illusion.
Candy is a useful character for exposition purposes. He spends his time around the bunkhouse and the other buildings because he is unable to work in the fields. All the other men are gone throughout the day, so Candy is there to greet George and Lennie when they arrive, to show them their bunks, and to provide information--in dialogue--which the reader and the audience need to know. He also represents the precarious situation of the men who get old and worn out, and he is useful for plot purposes because he has some money which seems to make George and Lennie's dream of owning their own farm realizable.