I don't know why people eliminate them its the samein Egypt mostjust bypass them rather than replace.That’s your/was brake proportioning valve.
It automatically adjusts brake bias front to rear based on load in the vehicle.
As weight in the rear increases and the vehicle squats an arm attached to the axle and the valve moves to adjust brake pressure to compensate for the weight to ensure even braking front to rear based on weight.
A lot of people eliminate them when they go bad. However, brake effectiveness may be compromised. An adjustable, manually, valve can be installed and through trial and error you adjust based on braking tests. You DO NOT want the rear brakes locking up, if they do there is the chance you will loose control as the rear of the vehicle will swing out!