Commuting, you win if you get to work, and get home again. Racing, you only win if you're first to the finish line. Over the course of say an hour-long crit or a 2-4 hour road race every acceleration takes a certain amount of energy. Racing is all about being in a position to try and win a race while conserving as much energy as possible so that you have more left in the tank than your competitors when that inevitable final acceleration occurs.