I don't think it has a purpose, but I think people wear it to make them an individual. Also sometimes I think people wear it to fit into a certain scene. Think about it, most people with lip rings are into rock or indie. When was the last time you saw a country boy or a sorority chick with a lip ring?
i like them, I was gonna get it done, but when I saw EVERYONE is getting one I'm like alright f*** that its not gonna be a "style for the moment" kind of thing for me I think its ridiculous. But anyway its like saying why do you get the type of clothes you wear? because it "defines" you in a way idk.