Floating down to Camelot

I find it strange that they actively forbid you from indicating when moving off if there's no-one there. If there's no-one there to see it, what harm can it do? In situations when indicating is appropriate, I indicate even if I can't see anyone, because there might be a pedestrian in a driveway, or a driver in a parked car who's about to turn the engine on, or whatever. Also, if I sometimes indicate and sometimes don't, it'll be easier to forget to do it even when I should. If I always do it, then it becomes ingrained habit and I'll never forget.