Balancing means compensating for both the weight of the tire & wheel after the tire is mounted . A wheel is out of balance when one area is heavier or lighter than the rest. This can cause irregular treadwear & vibration, & increase the stress on the front-end parts, which may cause them to wear prematurely.

You should have your wheels balanced whenever a tire is replaced, when a balance weight is moved or removed, & whenever you purchase new tires. Of course, at the first sign of vibration or irregular treadwear, your car should be thoroughly checked for wheel balance & alignment , & for worn or broken mechanical parts.