To limit the probability that foundations will be installed on loose soil which has a lower bearing capacity than expected, which could lead to the undue subsidence of soil beneath foundations, which could lead to excessive stress on foundations, which could lead to structural damage to foundations and 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.