The original manual style disc/drum master cylinders had a residual pressure valve for the rear brakes because the drum brakes need a little residual line pressure to keep the shoes close to the drum after you get off the brake pedal. So if you took one of those original master cylinders apart, you could remove the brass seat for the brake line at the master, and then would generally find a little rubber "duck bill" residual valve behind it. The dual masters you get today generally don't have the residual valve in them any longer. So installing an inline 10lb valve for the rear drum brakes is one way to keep the line pressure up so the shoes don't overly contract.
The proportioning valve is a little different in that too much pressure makes the shoes hit the drum too hard. So you can dial in a little less pressure to soften the hit so to speak. Really what you are doing is adjusting for the timing of the brakes to apply, and the amount of force. You want them to apply at the right time and with enough force to be effective but not lock up and become ineffective.
