The purpose for balancing the weight of a tire and its wheel assembly is that it travels evenly at high speeds. Balancing requires putting a mounted wheel and tire on a balancer, which centers the wheel and spins it to determine where the weights should go. Every time a wheel is first mounted onto a vehicle with a new tire, it has to be balanced. The goal is to make sure the weight is evenly distributed throghout each of the wheels and tires on a vehicle. This process evens out heavy and light spots in a wheel, so that it rotates smoothly. If there is even a slight difference in weight in the wheels, it will cause enough momentum to create a vibration in the car.
At high speeds, even a tiny imbalance in weight can become a large imbalance in outward force, which could cause the wheel and tire assembly to spin in a heavy and uneven motion. This usually turns into a vibration in the car as well that could cause uneven and damaging wear on the tires.
Maintaining proper wheel balance is a critical part of maintaining the life of the tire. Also proper wheel balancing ensures your vehicle has a smooth and vibration free ride.