To limit the probability of inadequate strength, which could lead to an inability to resist concentrated loads from beams, which could lead to the compression failure of hollow concrete blocks and the transfer of gravity loads to thin foundation walls, which could lead to foundation walls cracking.
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.