Similar to heavy wind, where a bad maneuver will provide immediate feedback in the form of a capsize, in light air it becomes painfully obvious when mistakes are made because the boat will stall out, and stop moving forwards. Small mistakes can cause large speed differences, and often times these same skills that are required to keep the boat moving in the light air are the skills that need to be refined in order to sail well in moderate breeze. By practicing in super light conditions, sailors can learn to perfect skills like boat balance, smoothness, and patience during boat handling, which will ultimately translate to speed across a wide range of conditions.
Next time you get a light air day where you are waiting for wind, take advantage of the opportunity to refine your techniques by finding a training partner, and working on speed and boat handling upwind. Work hard