from the wise Sheldon Brown, I have not seen anyone put it across better:
Why ride a Singlespeed?
Modern 24-27 speed bikes are marvels of technology, and allow a cyclist to select the gear ratio that will make the most efficient use of his/her energy. If what you're after is getting the maximum possible speed/distance for the minimum effort (and there's nothing wrong with that!) a multi-speed bike is what you need…but, efficiency isn't everything!
If you're riding for sheer pleasure, or for exercise, you don't necessarily place that high a premium on output results, as measured in speed, distance or vertical climb. Instead, you may care more about the actual experience of riding your bike. In this case, you may be a candidate for a singlespeed bike.
Riding a singlespeed can help bring back the unfettered joy you experienced riding your bike as a child. You don't realize how much mental energy you devote to shifting until you relinquish your derailers, and discover that a whole corner of your brain that was formerly wondering when to shift is now free to enjoy your surroundings and sensations.
Paradoxically, a singlespeed is, in another sense more efficient than a multispeed bike! While the single gear ratio will not be the "perfect" gear ratio for all conditions, in the conditions which fit the single gear, it is considerably more efficient mechanically than the drive train of a derailer bike.
A singlespeed bike dispenses with the weight of the derailers, shifters, cables, extra sprockets and longer chain. In addition, a singlespeed gear train runs the chain in a perfectly straight line from sprocket to chainwheel, and avoids the serpentine wind through the pullies of a derailer. You can really feel the difference! A singlespeed is noticeably quicker and easier to pedal than a multispeed bike in the same gain ratio.
Singlespeed bikes are also considerably more sturdy and reliable than multispeed bikes. There's no derailer to catch on the underbrush or to get overshifted into the spokes. The rear wheel itself is a lot stronger than one made with off-center (dished) spoking to make room for a whole bunch of sprockets on one side.