For added stimulation. Basically, because it feels good. Some like the way it looks too and think that the opposite sex will be more attracted to them.
1) Because it's a form of control and power to many; 2) They enjoy the pain of it; and 3) To many that do it, it's a pleasure to see it as they are completely and unconditionally into the piercings of their body; and lastly 4) It is also an artistic measure that many take into account from an artwork form of view. It's very attractive to most people and enjoyment is usually found in piercings.
Its not even painful during the sex activity but it helps to have more sensibility in the act and gives you more pleasure and if makes your self feel better who cares how looks its just like the people who have theme on their faces some of them don't even look good or attractive but that makes them feel good the difference its that the ones on the private parts no one can see them if you don't want.
Because they are fresh and think that is funner and better during sex especially they think that is attractive towards guys
Why does anybody get anything pierced? It's just another part of the body that can be!