To limit the probability of insufficiently thick concrete in foundation walls, which could lead to insufficient bearing of supported walls or eccentric loads on the foundation, which could lead to an inability to support gravity loads or lateral earth loads, which could lead to concrete 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 rough or uneven walking surfaces, which could lead to persons tripping or falling, which could lead to harm to persons.