I didn't say it was purely for hunting skill, it is partially for fun. The main factor though is that the parents are the ones that teach the kittens how to hunt in the first place. Everything a kitten learns from its parents within the first few weeks of opening their eyes is how to survive and how to kill. All those adorable traits and behaviours they exhibit all have their basis in hunting. The times they bring them to you dead is actually alpha behaviour in which they see you as dependent on them and they are supplying you with fresh food ^^ This is all covered in basic animal behaviour course.
But like i said playing happens too but it doesn't mean it doesn't have other purposes...