There's a variety of reasons.
Pain, for example, if applied properly and with a slowly building intensity helps release endorphins, which provides a non-drug induced high. I know many people who suffer from chronic pain who usually have to use heavy drugs. But when they want to take a break from the drugs, they'll seek out a good flogging, whipping, or even needles for the natural pain killer.
Some people like to play with control...giving or receiving. If you had the ultimate power in a relationship, what would you do with it? I know some people who use others up and spit them out as soon as someone new comes along. But I also know some who use their power to help keep the relationship beneficial and positive for both partners.
Submitting to your partner's desires means they tell you what they want, and how they want it without you having to guess nor worry if you are doing something right or doing it wrong. They will tell you. On the other end, telling someone what you want and how you want it means that you can get the kinds of activities that turn you on. And you can work towards clear communication and understandings of it without fear that your partner will leave.
Being tied up implies loss of not just control, but also of responsibilities.
For some, being tied up also implies being so greatly desired that you are prevented from leaving, that you are so greatly wanted that a person would bind you to them in some way.
Some people have high pressure jobs and when they finally get a chance to relax, they need a little help. Work hard, Play hard.
Some people like the psychologcal satisfaction of knowing that they have just survived something that would drop the average person. Some people use bdsm activities to work through emotions, trauma, etc. Some treat it as a form of artistic expression. Some just think it's fun and not to be taken too seriously.
There is such a huge range of psychological factors and sensation interpretations involved, often even in just one person. There are some books that go into some of the psychological aspects, but even those must narrow their range due to size limitations.