To limit the probability that the vertical bar reinforcement will not be adequately encased and bonded to the concrete, or of excessive eccentricity of load resistance, which could lead to the cracking or displacement of foundation walls.
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.