You've gotten two very good answers to your question; I only have one other element to offer which might add to the discussion.
Lennie and George have a dream--a place of their own, a farm where Lennie can tend the rabbits. It's just a dream, because they each have something (besides their obvious lack of money) which is keeping it from coming true. Lennie would not be able to raise rabbits, as he is a compulsive "petter" and would eventually kill them. His handicap here is his strength. George's handicap is Lennie; he would forever be trying to keep Lennie safe and then trying to repair the damage when he failed.
Candy is perfectly suited to join this venture. He is unhappy and feels unproductive, so anything to look forward to with hope is a blessing to him. We know, though, from seeing what happens to his dog, that Candy will not be part of the dream. If he can't keep them from killing his dog, he certainly won't be able to assert himself in any meaningful way. His handicap is not his missing hand; it is his inability to be more assertive and useful. He lives in fear, and that fear will keep him from any kind of forward movement.
When Lennie dies, the dream dies. George might still one day achieve his dream, but it seems unlikely since it was what George wanted for Lennie more than for himself. It's certain Candy will never leave the ranch. Hope is dashed and the dream, for them, has died.
But that doesn't mean dreaming has no value. It kept Lennie alive until the very moment he died.
After Candy suggests joining George and Lennie in the plan to have a ranch,
They fell into a silence. They looked at one another, amazed. This think they had never really believed in was coming true. George said reverently, "Jesus Christ! I bet we could swing her." His eyes were full of wonder. "I bet we could swing her," he repeatedly softly.
This is a very significant point in Steinbeck's novella, Of Mice and Men. For, it underscores the theme of the fraternity, the communion of men. Joined together, men can overcome their alienation and live peaceful and happy lives that have meaning.
This dream of fellowship has been explored as the solution to the innate alienation of man as George and Lennie arrive just south of Soledad, a name that means solitary. Steinbeck knew that man's condition is that of loneliness, but, as a Socialist, he hoped that a union of men could mitigate this condition.
This quote from Of Mice and Men is important for a couple of reasons.
First, it demonstrates the prevalence of the dream of owning one's own place and working for oneself. George and Lennie are not the only ones who dream of working their own place. Others dream that dream, too. This ensures that the poverty and the difficulties the poor face are seen as universal, not just limited to George and Lennie. This extends the novel's revelations about existence beyond just George and Lennie.
Also, Candy's financial contribution gives some substance to the dream. Owning a place of their own actually seems possible for George and Lennie, with Candy's help. This makes it all the more devastating when, later in the novel, the possibility of the dream falls apart. Steinbeck creates a dream that is seemingly within reach, then destroys it. If the dream is completely impossible and out of reach, its loss is not so devastating. Candy's contribution accomplishes this for the novel.