He uses Candy as a tool to continue his use of the theme of fraternity, which means a group of people sharing a common profession or interest.
Steinbeck uses Candy to state that society in the 1930's was an extension of life on the ranch in the novella. Each person is to embody a group of people in the larger society, for example, Candy embodies the elderly and disabled, yet not hopeless portion of the society, Slim is the successful big-shots who have made it in the world, Crooks the foreign asylum seekers who were/are discriminated against, etc.
We see that Candy is not hopeless when he talks about 'the dream' with Lennie and George, he has hopes to go to the dream farm with them, the dream in the novella being an embodiment of the American Dream.
This, in conclusion, shows that Candy is used by Steinbeck to show that the elderly and/or disabled portion of society weren't all lost causes, and that some still had hope in themselves or in the American Dream.
