The proportioning valve divides up the braking effort applied to front and rear wheels under heavy braking, according to how load is distributed across a vehicle.
The effectiveness of braking force is determined by tire-to-road friction. And this increases as load increases.
Applying the brakes causes the front of this vehicle to dip. This causes greater tire-to-road friction on the front tires, and less on the rear. This kind of change of load is called load transfer.
So, if equal braking force is applied to the front and rear wheels, the smaller rear load can make the rear wheels lock, and perhaps skid.
The braking force applied to the wheels needs to be adjusted to allow for changes in load.