Think he basically provides you a symbolism of the broken American dream. Also, how the society is broken and not dealt in a meritocratic way since Curley is so useless but born into wealth. You can further see his broken hand as a visual metaphor for what he was to the society; just labour. Without his hand he's worth nothing.
You can get really contextual about it and say if you look at other cultures, for example, immense amounts of respect is placed on the old however in America it is not. Maybe Steinbeck was trying to focus his efforts on stressing what capitalism does to the society. Despite Candy's experience he is useless cause he can't do labour.