it is NOT required to have it on all cars.
If a car is especially heavy at one end, it will cause the car to push or oversteer (heavy in the rear causes oversteer, heavy in the front causes understeer).
If your car has a strong weight bias (FF is front biased, MR is slightly rear biased, and RR is more rear biased...) you can add weight at the opposite end to adjust the car's handling balance.