They're for people who feel insecure, so they add details to their body to feel more interesting. It's like people who buy tons of accessories like rings and watches. Recently I've begun to view a lot of things people own/do this way. Tattoos, accessories, titles, hobbies, ideologies, religions, money. All an attempt to build an identity for yourself. (It's not exclusively this way, some people may really believe in certain things, but I think if it doesn't come from said insecurity, it might come from brainwashing)