Many reasons, just to name a few
1. For a change. It's different from kiteboarding and a lot of fun!
2. I prefer sitting over standing sometimes
3. I like to take passengers sometimes(e.g. my 8 year old son)
4. Many people are being fascinated by power kites, these are not only used for kitesurfing, but as well for buggying, landboarding, kite-skiing. Why not for powering a boat?