Benefit of high belt drive tension?
Don't have a belt drive myself, but a guess would be:
Running appropriately snug (I assume they crafted the app for a reason) would make sure that the stresses and load on the belt is distributed appropriately. Consistent tension on the belt has to be better than a looser belt undergoing stress fluctuations as portions of the belt change from a loaded state (on/between the rear hub and the front crank) to an unloaded state. I expect that higher stress fluctuations on a looser belt would create additional 'belt fatigue' and reduce the belt life.