Single speeds are interesting. One gear. That's it. Some will argue that you have two actually; One is riding, the other is pushing your bike. Makes sense. But why would you want to ride a bike with only one speed, especially with the modern advancements in bicycle shifting?
Bicycles are a great invention, and for the most part are very basic. The only exception to this rule is the drivetrain. Derailleurs can be finicky, one slight bump and its out of alignment and shifting poorly. Front derailleurs get clogged with mud and stop working sometimes. And then there is the dreaded chain suck. Not as simple as it should be. That's why single speeds are important.
One speed. Just you and the bicycle. Want to ride faster? Pedal faster. Riding a single speed is a whole new cycling experience. Now instead of thinking about shifting, the only thoughts are about the trail. How the ground flows, where can you use your momentum to your advantage, etc. Instead of sitting to spin up a hill in the granny gear, you are out of your seat grinding out the climb. Riding a single speed changes everything.
I am a big fan if you couldn't tell already, and after not having a single speed for around a year I am glad to have one again in the stable. If you have never given it a shot, try it out. It will surprise you on your first ride, and probably stick with you for a while.