It is necessary since you don't want the front and back using the same pressure. The valve gives a bit more favor to the front brakes and a little less favor to the rear. Even with stock diameter brake lines. That is how I understand it, but someone else chime in please.
Residual valves are 4 pounds for disc and 10 for drum. They are not necessary for drums all the way around. They keep just (said amount) of back pressure on the system so its ready when you are. I had to put them on my disc front/drum rear setup.
