Let me remind you, if you can’t handle it, please, don’t click and don’t comment.
For a quick “why”, some people like to push the sensations of their sexual experiences farther than what vanilla sex can provide, and to do so, one has to figure out how to integrate pain and all other sensations and emotions into the mix to get the most intense experience. I always wonder why people have trouble understanding that or get upset when they’re exposed to it — it’s not as if the concept is alien in other arenas, most obviously things like extreme sports (freeclimbing for example), and of course most body ritual (suspension and so on) also gives a hint of the same phenomena as well.