If they're in a gang or criminal syndicate, they'll most likely serve as the "girlfriends" of gang members and will kill, steal, cook for, clean up after, take care of, and have sex with them whenever they need it.
If they're in the Mafia, they'll either be "wives" or "honeys" of hitmen and will take good care of them, wait on them hand and foot like servants, fuck or "service" them, cook for them, fill in for them when they're sick, act as marksmen during heists, bail them out of jail whenever they get busted, and basically be the bitch and slave to all members of the Mafia except the Boss.
Hitman #2: There she is now.
Hitman #1: Hey, toots. How about youse let me bang you till you moan and scream?
Henchgirl: Yes, Alfonso.