If you really wanted to know, I could respond to this at length (it is something I've done research on, as have many other people). The short answer is: People, in cultures where it is done, may do it for a number of very different reasons. If it is not necessary or useful for somebody, there is no inherent necessity for it. You are not alone in finding it useless or puzzling. Personally, since I've worked so much with it, I kind of wish it were an option here because I do see uses for it for myself.