Goodfellas summed up the mob's purpose the best. They settled problems for people who could not go to the cops. A police force for wise guys.

Don Corleone was a fictional character. As LittleNicky mentioned some guys had a charitable nature, but lets be honest, most of these guys did that to help gain support from the same neighborhoods they were robbing. Just like Nicky Barnes used to hand out turkeys to the same people he was killing with smack. Sure on a personal basis some of these guys are fun to hang around and even friendly, until you cross them. Maybe there was a time, probably in Italy, when the mafia had altruistic motives but that was probably several hundred years ago if it was ever like that at all. The only good side of the mafia is they are fun to read about and they help sell newspapers. That is the only legit contribution they make to society. I realize some of these guys have legit or have gone into legit business. I am speaking about those that are still involved in criminal activity.