To keep the unsprung weight down. Unsprung weight is the weight of the car that is not supported by the springs. The more unsprung weight you have, the less effective your suspension is. Take the original Clodbuster for example. Most of the weight of that vehicle is below the springs... Differentials, motors and servos.. The only thing that's supported by the suspension is the body, chassis and batteries. What's doing most of the suspension work is the tires... In a race vehicle like a XX4, B4, BJ4 or whatever, the only unsprung weight is the arms, tires and a couple bearings at each corner. With all of the weight on top of the springs, the suspension does all the work it's supposed to making for a much better handling car.