Well, it doesn't in the game (in race situations anyway). And I don't work by quicktune menu descriptions, I take the car to the track and try which setting work best. Try the R35 GT-R, set Physics to PRO, ABS to 1 and set the brakes to 1/9, take it for a few laps on Suzuka, then try the same with the brakes at 9/1.
Your question and its answer don't mean a thing. Seriously, you don't understand that the powers at work and weight transfer in both situations are completely different?
I agree on the testing but I have to desagree on pushing the brakes above 7 or 8 on any cars setup. I find I get to much lock up even with ABS set to one. You can tell when you get lockup is when your wheel skips. ( slight turns in straight line or turn braking) You'll onlu be able to fell this in the wheel if your force is set to 10 and power-assisted steering off.
Very true I forgot about that.
I mentioned those settings as an easy example to show what effect they have in game. By no means use them in a race.
True, ABS prevents the wheels from locking, so you get a nice oversteer effect rather than a horrible spin. With ABS off however, the horrible spin is still an effect of rear wheels losing traction = extreme oversteer, causing the car to spin out.
So the way you tune stays the same, it's just that the settings will be less extreme (you can get away with a 9/9 brake setting using ABS, try that with ABS off 
). And I have tuned with ABS off in the past, but since everybody is using ABS=1, I no longer do that. 
unrealistic or something?
When we get private rooms do you think people will be more interested in racing each other with no one using ABS???? Im pretty sure most of the people that race the pro physics events would much rather have the physics be more real, and as you said the ABS on 1 makes you able to do things that you couldnt do in real life. Why is it people race with the ABS on 1 anyways? Is the braking in GT5
In addition to that, your body does not experience the physical deceleration force in GT - in real life, this sensation is a great calibration tool for brake application.
Brake bias wont naturally induce under or oversteer if you stay within the limits of the tires, because braking distribution doesn't affect weight transfer (suspension and centre of gravity height affect this).
However too much front bias means if you are heavy under brakes, the front tires are on the limit of traction, so if you then try and turn, the car wont want to turn, so you induce understeer by the nature of you have used up all the available grip.
If you have too much rear bias, the car will be unstable under brakes. Remembering the car will go where the rear wheels point (as they dont change alignment with the chassis but the front wheels do when you steer). If you have too much rear bias the rear wheels wont have enough traction to keep the chassis straight so any disturbance will make the car unstable. This is kinda like oversteer.
Doing donuts, the only thing brake bias has to do here is keep the car in position while you shred the rear wheels. If you had rear bias, the brakes would slow down the rear wheels which stops them spinning... donuts are not "oversteer" in the racing sense
Put simply, think of it like allocating the tyre's available grip. Let's say, by complete random out of the air numbers, the front tyres allocate 70% of their total available grip to braking, and the rears 50% of their available grip to braking (while braking), at a brake balance of 5/5, which would actually= 100% available stopping power from the brakes themselves. By moving that bias towards the rear, say now the front uses 60% of its grip for braking as does the rear, then the front tyres now have more available grip for turning (40% of its available grip) which means the front tyres can bite into the corner better. That's why the brake balance works the way it does.
