To limit the probability that organic material will decompose beneath foundations, which could lead to soil having an inadequate bearing capacity, which could lead to the undue subsidence of soil under footings, which could lead to the cracking of foundations or the deformation of building superstructures.
For floors and elements supporting floors, this is to limit the probability of excessive deflection or damage, which could lead to rough or uneven floor surfaces, which could lead to persons tripping or falling, which could lead to harm to persons.