To limit the probability of insufficient bearing area for foundations, which could lead to compressive failure and settlement of the underlying soil, which could lead to structural damage to the foundation and the building, which could lead to an inability to support vertical building loads or lateral earth loads.
This is to limit the probability of:
For floors and elements supporting floors, this is to limit the probability of excessive deflection, which could lead to persons tripping or falling, which could lead to harm to persons.