The residual valves maintain a minimum constant pressure in the system, for disc brakes it keeps enough pressure to keep the caliper full of brake fluid but not enough that the brakes drag. If they are not installed your master cylinder will not have enough capacity to apply the brakes with one stroke of the master cylinder.
The proportioning valve is used to set the amount of fluid that is sent to the rear brakes vs the front brakes. It helps prevent rear brake lock up. On performance vehicles a proportioning valve is used to adjust the amount of brakes to either the front of the vehicle or the rear of the vehicle. It is used as an aide to get a car through the corners faster by helping steer the vehicle through the corner by using the brakes.
