To limit the probability of insufficiently wide strip footings, which could lead to compressive failure and excessive settlement of 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 rough or uneven walking surfaces, which could lead to persons tripping or falling, which could lead to harm to persons.