The difference between the standard load valves and a bias valve are...

The standard load valve are mounted on the body with a link arm onto the rear suspension (one on each side/circuit), when you press the brakes the rear of the vehicle wants to go up meaning the rear wheels have less grip. If the load valves were not there or incorrectly adjusted the pressure would not be reduced to the rear brakes and they would lock up.

The bias valve goes into the pipe to the rear brakes, it reduces the amount of pressure to the rear brakes by limiting the flow or brake fluid (like standing on a hosepipe reduces the flow of water). The valve is adjustable so you can set the balance or 'bias' between the front and rear brakes.