My reasons:

Different leg muscle training (it also forces me to push more during climbs and has helped my spin with descents)
I enjoy the difference in riding technique
Feels more challenging/rewarding when undertaking longer/harder rides
I like riding bikes, my fixed gear is a bike, I like riding my fixed gear bike.

Try it if you're curious, if you aren't - don't. If you like it, you like it. Simple, really.