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.
Where assemblies are required to provide fire resistance, this is to limit the probability of compromised fire resistance of the assembly, which could lead to harm to persons.