I look for hills. As for "reducing your bike's functionality by stripping it down to one gear"... well... first off, you only need one to make it go, and even with 30 gears you can only use one at a time anyway. They're generally lighter, stonger, cheaper, and less finicky than geared bikes. There's no thinking about being in the right gear because you're always in the right gear, you just pedal. For me the big reason tho... it's just fun. I think the real question here isn't "why singlespeed?", it's "why multispeed?"