Racing vehicles not only sport high-performance brakes but also allow to distribute the braking power to the front and rear axis according to the driver's needs and preferences. Why would anyone want to play around with these values?
We all know from experience that weight is transferred to the front under braking. Applying this knowledge on a car with two axes means that the front axle will have to cope with a bigger deal of the car's weight while braking than when driving freely. Equally, the weight on the rear axle will decrease under braking, thus making it 'lighter'. It is because of this weight transfer that the braking pressure is directed mainly towards the front.
The problem with putting the brake balance towards the front end is that the front tyres have to divide their grip on lateral forces (steering) and longitudinal forces (braking). The harder you brake with such a front-biased setting, the less you can steer [This is what is meant with the term 'traction budget'] . This means that you can get understeer regardless of your wing-setting ! Putting the brake bias more to the rear will leave the front tyres with more grip to use for steering so that the car will better respond to your input and turn into the curve. Bear in mind, however, that now the rear tyres have to cope with higher longitudinal forces (braking) which leaves them less force to keep the rear from breaking away - the car will tend towards oversteering the more the brake-bias is set towards the rear. [This is not a bad thing by itself but it has to be mentioned here.]
This means that you can in effect intensify (or mitigate) the car's tendency to under- or oversteer by moving the brake bias to the front or rear. [Naturally this only works in curves for which you have to brake and steer. It does NOT affect the behaviour if you do not step into the brakes or brake in a straight line.]
Adjusting the brake balance is more than just fine-tuning the car. It has a very strong effect on your driving. Only a good brake balance that is attuned to the track will allow you to use the full potential of the car. Experience shows that F1GP strongly favours setups with rear brake bias (see additional notes). The exact value depends on the wing setting as well but the usual range goes from 5R to some 24R or even higher. A good setting for most tracks is 12-14R (except for Monaco and Magny-Cours) since the car's handling is still manageable and yet clearly shows the advantages of such a rear brake bias.
Moving the brake-bias towards the front (for example from 16R to 12R) will make the car stable and predictable which is good for a long race but may cause problems with understeering at the curve-entry when driving a light car like in qualifying.
Putting the brake-bias towards the rear will cause the car to tend towards an oversteering behaviour - the car will eagerly turn into curves so that you can slide around the apex very closely and apply the throttle early which makes it perfect for a fast qualifying lap.
If you regularly fly off the track by driving in an almost straight line when entering a curve after a braking manoeuvre, try putting the brake bias more towards the rear. [Assuming of course that your speed is more or less suited for the curve]
If the car turns into the curve so fast and abruptly that you actually drive over the inner curbs although you do not want to, then you have put the brake-bias too much towards the rear. Try to balance the car by moving the brake-bias a bit towards the front. [There is a way to keep driving with such a strong rearward brake-bias - start steering later and rely on the quick oversteer-snap to get the car round in the last moment but that is not an easy manoeuvre in my opinion.]