
Some people pierce their nipples for art, for politics, for pleasure. You get the picture...
Then there's also BDSM nipple play, where some delightfully painful "earrings" can put a clamp on the fun and flow of things. Really--don't they look like earrings? I mean, you really can clamp them to your ears. See below.

If you're a newbie to nipple clamps, the first sensation is pain. Eventually, your nerves grow numb in the area. When you release the clamps, the sudden flush of blood goes to the area and the person feels pain once again. This sensation can be so intense for some that they feel a direct connection to their sex organs. Go figure. :-)
Best way to find out is try these "earrings" yourself. ;-)