I'd like some clarification too. Is there a point using anything other than front bias? Braking into high speed corners would benefit from rear bias?
Brake bias forward: Shortest stopping distance, but car wants to stay straight while braking
Brake bias neutral: somewhere in the middle
Brake bias rear: Longest stopping distance, but car can turn under braking.
More in depth: Braking causes the car to pitch forward, which reduces grip at the rear of the car, and increases grip at the front.
If you are traveling in a straight line you want to set the bias far enough forward so that all 4 tires lock up at the exact same time for optimum stopping power. If the fronts lock up before the rears, your rear brakes aren't strong enough, and vice versa.
If you are cornering and braking you want to set the bias more toward the center so that your front wheels don't lock up, they are already heavily strained by the cornering forces and if they lock up you'll just understeer off the track. keep in mind that your front tires steer, and your rear tires drive.
Locking up the rear tires is just bad news and usually causes your car to spin out. This is because once the rears are locked they want to keep going in the same direction, while your front end is still gripping and slowing.
At least that is the explanation that I received from
John Robertson