Besides being "As strong as a bull" and able to do pretty much whatever you tell him, and able to protect George from anyone who might do him harm (like we see during the scene in the Crooks' quarters when Crooks merely suggests it), Lennie also provides George, a homeless and migrant worker, with a sense of being needed. George has someone to take care of, someone he is brighter than and someone who needs him. He has no wife and no children that we know of, but he has Lennie.
Lennie is also a key part of his American Dream - the idea of settling down on their own land, free from the threat of being "canned", and just forgetting about work when it rains would mean little if George didn't have someone to share it with. The dream itself was so fragile, without Lennie to hear it and share in his excitement about the possibility, it is doubtful the dream would exist at all.
While George complains in the first section of Of Mice and Men,
"I could get along so easy and so nice if I didn't have you on my tail. I could live so easy and maybe have a girl"
toward the end of this same section, he amends his words,
"With us it ain't like that. We got a future. We got somebody to talk to that gives a damn about us. We don't have to sit in no bar room blowin' in our jack jus' because we got no place else to go. If them other guys gets in jail they can rot for all anybody gives a damn. But not us.
For Steinbeck, who idealized male friendships, the frienship of Lennie and George portrays the way to overcome the terrible loneliness that pervades their world. In addition to having a friendship with Lennie, George is also protected by the strong Lennie as well as enriched by Lennie's presence since, as a child-like man, he is the keeper of the dream of owning their own ranch, he it is who makes the idea of having a future possible. For, George merely recites the dream to comfort Lennie, but he repeats it so many times and invites old Candy into it until it become a real possibility. Once Lennie dies, however, the dream dies, too. Old Candy knows this as he looks down at the dead wife of Curley:
"You done it, di'n't you? I s'pose you're glad. Ever'body knowed you'd mess things up."
Lennie provides unconditional love and support to George. Although, George appears to be the primary caregiver, Lennie is a friend to George, who often finds himself lonely in the world. Lenny is George's sidekick, for sure, but he is also a good listener, one who would never repeat George's secrets and dreams--a confidant, so to speak.