Gearing matches your power output (in watts) with your ideal pedalling cadence (revs/min) and the drag on the bike (caused
by air resistance, gradient, surface roughness, load carried).
The range of gearing that you need should suit your very best and very worst scenarios. If you carry big loads up steep hills on rough tracks, you need a lower gear than if you ride unladen on the flat.
The intermediate gears should be close enough that you can chose your ideal cadence under any conditions. More gears let you get closer to your optimum cadence but there are often many dupliate ratios and unusable ones. A rider on a flat time trial will want a closer selection of steps than a tourist who prefers the gears spread out into a wider range.
Triples permit a wider range of gears with acceptable steps.
Doubles are lighter, narrower and shift more cleanly.
Std doubles are ideal for racing athletes. Slower riders dont use the big gears and run out of low gears, hence the increasing popularity of compact doubles.
Single chainring setup are used by flat-land riders who need more than a singlespeed.