Not only is spinning easier on the knees and better for cardio and endurance, it also gives you an advantage when accelerating. Track racers are constantly fighting to find the balance between a gearing high enough that they can still spin at high speed but something low enough to let them jump at moderate speed.
It is much easier to jump to a sprint from a moderately high cadence (90-110rpm) than it is to jump from 60rpm. The exception, of course, are sprinters, who have to basically compete in a drag race on the last lap. But they are able to use the bank of the track to help them get on top of their gear.

On the street, I noticed a big difference when I lowered my gear from 48x16 to 50x18. For 90% of my riding it was not only easier for me to stop, but also easier to accelerate from moderate pace to a sprint to make gaps and avoid bad situations. For me, my gearing choice is much more about this than about climbing hills.