To limit the probability of an inadequate diameter of screws, which could lead to inadequate capacity or rigidity, which could lead to an inability to resist expected dead and live gravity loads, which could lead to fastened elements detaching from supporting members, which could lead to excessive movement, deformation or deflection.
For floors and elements supporting floors, this is to limit the probability:
This is to limit the probability of excessive deflection or vibration of floors, which could lead to persons losing their balance, tripping or falling, which could lead to harm to persons.