Andy F is correct. The proportioning valve limits maximum pressure to the rear brakes so they don't prematurely lock-up . Once the rear lock-up in a panic stop it can cause directional instability (car goes sideways).

With a properly designed 4-wheel disc brake set-up (disc sizing and friction material) and a properly balanced vehicle, a prop valve is not required and may increase stopping distance.

A further consideration is that you may already have a prop valve incorporated in your combination valve. If they car came from the factory with front disc and rear drum, and the car has the factory combo valve, then it's possible a prop valve is already installed.