In terms of efficiency, the human animal is good at a steady output rate.
We tend to be better at a relatively higher cadence and lower power output, so its no surprise that we make good long distance trotters compared to most other animals.
Having one gear on a cycle means you are frequently either pushing hard, or you are over revving. Most times you see fixed wheel riders on relatively flat terrain where you can keep more or less the same pedalling rate - there are exceptions such as track sprinters but then they do not typically ride for 50-100 miles at a time.
When you get to road racing, events can easily get into the 70 mile range, and for the elite riders frequently exceed 100 miles, especially for title events.
The gears are much more about controlling speed in the case of such riders, road racing is very strategic and the idea is to use what you have available in the most effective way, so you might try to shed out a few riders early on by riding the larger gears and go at higher speed, and then settle down until you reach race changing places such as climbs etc.
The cadence usually keeps within a fairly narrow band for the most part, but the speed will vary as differing gears are used. When you are riding close to the limits of your personal endurance, then .5 mph - which might be just one tooth difference on your sprockets, can be critical.
The thing is, as a rider your abilities can change significantly race by race, so that you find that you can ride slightly faster one day than another so you need a good selection of pretty close gear ratios, but the range of ratios available still needs to be pretty wide to cope with all the conditions, which means having lots and lots of sprockets.