To limit the probability of an inadequate footing width, which could lead to reduced friction among soil particles, which could lead to an inadequate bearing strength of the soil beneath the footings, which could lead to compressive failure and excessive settlement of the soil, 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 rough or uneven walking surfaces, which could lead to persons tripping or falling, which could lead to harm to persons.