Sorry, didn't tell you why.
Drum brakes rely on a one way seal at the cups in the wheel cylinders. The springs pull the shoes back. There is a possibility for the seals to relax when there is no pressure applied to them allowing air to enter the wheel cylinder. The next time you apply the brakes, you have a spongy pedal due to the air in the system. A 10 psi residual pressure valve keeps just enough pressure in the wheel cylinder to keep the air out.
Disc brakes rely on a square cut o-ring to seal the caliper. The flex of the o-ring will pull the piston back a couple thousandths of an inch so the pads do not wear when the brakes are released. The square cut o-ring seals equally inside and out. A residual pressure valve would cause the pads to wear prematurely.
When you have an under the floor master cylinder on disc brakes, the calipers can be higher than the MC. With nothing to restrict the fluid, it can flow back into the MC, pulling the caliper piston back into its bore. The next time you hit the pedal, it goes to the floor. A 2 psi residual pressure valve is enough to prevent the back flow but is not enough to keep the pads applied.