Think of usable gears in terms of how much effort you spend to travel a certain distance. If one turn of the crank moves your bike 25 inches, that's a very easy or low gear, if one turn moves your bike 100 inches, that's a very hard or high gear. The reason to have so many gears is the spacing between those 2 numbers. Some people go as low as 20 inches to climb hills loaded. Roadies like 110 inches plus to sprint.

Then there's the question of overlap and spacing. A good spacing is 15% between gears, but many gears overlap on a 3 ring crank as was mentioned, so you end with fewer actual "steps". You also want to avoid too steep a chain angle to avoid chain wear - smallest front ring to largest rear cog and viceversa. This is why internally geared hubs are so appealing. A single chain ring, single chain line and all usable gears in the back, one after the other, without having to chase the different combos. Look up Rohloff hubs on Google to see the ultimate in gearing - 14 serial gears without duplicates!