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 . . . . . I had to put them on my disc front/drum rear setup.
I do not agree, if the brake system is original specification: the different original-diameter brake lines (front and rear) and different original diameter-bore wheel cylinders (front and rear), provide appropriate/adequate proportioning.
