To limit the probability of insufficient reinforcement or that reinforcement across a cold joint will not be adequately bonded to the concrete, which could lead to tension loads not being transferred to each side of the joint, which could lead to concrete cracking or the 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.