For coarser soil types near the footings' bearing surface (for which free water between soil particles can reduce bearing capacity), to limit the probability of excessive soil subsidence, which could lead to the cracking of foundations or building superstructures.
This is to limit the probability of:
For floors and elements supporting floors, this is to limit the probability of excessive deflection or vibration, which could lead to a loss of balance, tripping or falling, which could lead to harm to persons.