Why, during sparging, do all-grain brewers drip water over the grains instead of simply filling it up with water again and redraining that?
It's simply a different technique. Batch sparging (filling up the mash tun and then draining) versus fly sparging (dripping water while simultaneously draining) is a long standing debate where folks like our own Denny Conn will side with batch sparging, while others will take the side of fly sparging. They both generally yield very similar efficiencies, so to say one is better than the other is often moot.
The trend that I see the most is that if a brewer is using a level brewing stand (single tier) with pumps, more often than not they'll utilize fly sparging. There are exceptions to this of course, but more often than not, if pumps are used, brewers will often use the fly sparging technique where others who don't use a pump will batch sparge.