There can be different points for different people.
Some people absolutely love the pinch they provide. Some people like ones that provide an intense bite, whereas others just want ones that provide a gentle bite. Some people just love the way that they look.
Some people really like inflicting pain, and/or having pain inflicted upon them. Other people don't enjoy it at all. Some people get into controlling or being controlled and pain toys like nipple clamps are another way to do that. And Bellefleur's right: they do look sexy!