I've got three bikes. One's a commuter with 21 gears, one's a road bike with 20 gears, and one's a mountain bike with one gear.

For commuting the 21 gears come in handy. Biking around town, I don't ever use my lowest chainring (22). I usually keep it in the middle (32) unless I'm on a flat road long enough to get up to speed, then I'll switch to the largest chainring (44). The middle chainring + the 7 gears in the back would be enough to get around. However, I like having the big chainring to go even faster while keeping the same pedaling speed (cadence).

I have to use my smallest chainring (22) when I'm biking up the hill I live on. It averages 10% in grade for a third of a mile.

For the road bike, it's obvious why there are so many gears. Your speed will vary a lot while climbing to descending. Road biking is all about efficiency by keeping the same cadence using your gears.

I'm not sure why I chose a SS mountain bike. I think it makes more sense than a SS commuter or road bike (unless it's flat). The gearing on the mountain bike is low enough for steep climbs and/or long climbs. It's high enough for me to spin up to a decent pace, unless it's a nice flat smooth trail. Once I hit a certain speed going downhill (near my gearing limit) pedaling will only make a small difference.