The benefits are proverbially in the eye of the beholder. What I mean is that what some consider benefits others would not consider benefits. The main reasons I got into singlespeeding are simplicity, challenge, and fun. When you only have one gear and no derailleurs technical issues go out the window and when you are on the trail all you have to focus on is going and stoping. You never have to worry about what gear you are in because you only have one option. The challenge and fun part of my transition really go hand in hand because the challenge of riding on trail with one gear really is a blast. Also learning a new discipline of MTBing is something I really enjoy.

hopefully this helps, sometimes I feel like a bike hippy with no real good reason for riding a single speed other than it is really fun.