Knowing cadence is to cycling what knowing rpms is to driving. Cars work perfectly fine without a tach, but it's harder to know exactly what's happening. A guy who raced some classic (i.e. old) car series said that he knows when to shift when the whole gauge cluster shakes so hard he can't read anything. Apparently he worked out the speed and that's about the limit of the engine.

Work, especially going up hill (since you rarely draft etc) is the same for a given speed/grade/wind/rolling resistance. In other words, if you're lugging along at 30 rpm at 5 mph, you will do the same work if you pedal faster in a lower gear at 5 mph.

Knowing your exact cadence, without counting for 6 seconds and adding a 0 etc, is helpful when trying to optimize your efforts.

Higher cadences tend to load the muscles less, leading to less "dead" feeling legs (lactic acid). A typical racing trick is to use lower gears for the less important parts of a race, then to use bigger gears as the race gets hard. I've done this accidentally, racing in the small ring for all but 1.5 laps of a race, but I got into the big ring too late. A friend does this regularly (he's a much better rider, a Cat 2), and he'll do half of a race in his 39 or 42, then "gear up" into his 53. Averaging 25 or 28 mph, with bursts of 30-35 mph, in a 39 or 42 is pretty tough. However, your legs don't feel as fatigued when you put it in the big gear, and in fact this practice really encourages you to spin.

One old trick you can try is to use a bigger gear for a bit, observe your speed, and then, after you've acclimated to that gear, shift to a lower one. It'll feel easier of course, but oftentimes your speed will actually increase.

At the same time, it can be useful to roll in a big gear at lower rpms (60-ish) to develop strength in less-used muscles, or less stressed muscles.

Of course when you're struggling on a hill and you explode, you end up going at whatever you can go - 45 rpm in some gear for example. But knowing that it'd be better to maintain a slightly higher cadence may encourage you to let go of the gear you're on and shift down.

cdr