Well to keep it from floating on rising ground water
I'd never heard of this before, then I caught a show called Zombie House Flippers - zombie houses, not the flippers, themselves ;-p - where one of the homes had a pool where they were worried about this very thing. They had to empty it to repair it, and it had been raining a lot, so they wanted the repairs and refill done as quickly as possible. They were somewhere in the southern part of Florida.