What role do minor characters play in a piece of literature?

In my opinion they encourage the story to move on. Minor characters include friends, family, loved ones...things like that. These people all encourage the protagonist(s) to move forward. I mean, if the protagonist didn't have any of these, how are you going to be interested in an antisocial person who doesn't have a family and doesn't love any other living soul? (No offense to those of you who do that, this is just an example!)
I hope I helped! ^_^