Alot of people use the creation of .iso's for the backup of thier games due to the fact that they dont want them to get scratched. Then you get more shady purposes when people cant be arsed to buy the game and just download the .iso instead. Or buy the game, take it home rip it and take it back to the store the same day and change it for something else.
But Yeah, backups are just handy if you dont want your 2 year old niece using your discs as chew toys.