I didn't read above so disregard if I duplicate anything.
It's basically a choice between overall range and smaller steps in between. You have X number of gears, trying to get more range than you need means that you'll have wider spacing between those where you do most of your riding, and will be limited in the ability to fine tune for small changes in conditions, like small grade or wind changes.
Most people mis adeaute low gearing more than anything else, so this should drive decisions, but you don't want to gear for the Alps, if all your riding is in Kansas (analogy only, don't tell me about those steep walls in KS)
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