Watch any pro mountain biker riding, and one thing they all have in common, other than being supremely fit, is that they have the ability to generate an impressive amount of speed in just a couple of short, sharp pedal strokes.
And while having a good start/finish line sprint has obvious advantages for racing, there are lots of situations on the trail where being able to put down the power with a few effective pedal strokes can make a massive difference to the outcome.
Like maintaining momentum when cranking up and over a technical rooty section on a climb, or simply smuggling in a few extra pedal