There are as many reasons for modifying one's appearance as there are people who want to do it - aesthetic, fashion, or maybe identification with a group. Often, visible piercings go with other forms of body adornment, such as tattoos. It is becoming mainstream. Some people who originally got pierced for the punk shock effect are vexed to find that the bounds of acceptability have spread to embrace them. Piercings can be used for restraint.
Some women lace their labia together.
One I heard of had a zip fitted. Men have other options. You may have it done for your partner, but be sure you trust him or her. He went first. As soon as he was done, she laughed and walked out.