To limit the probability that the vertical reinforcement will be interrupted at critical transition points or that it will be insufficiently embedded to develop a bond with the concrete, which could lead to an inability to support transverse or vertical loads, 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.