Tires and wheels are pretty heavy. When a tire is mounted on a wheel, it is usually not perfectly balanced. So the tire technician will spin the tire on a machine to determine where it’s too heavy. He will then place weights on the wheels in strategic locations to balance it out.
Most high-quality tires hold their balance pretty well. They just get out of balance gradually with normal wear and tear. If you suddenly feel a vibration, it is probably because you lost a balance weight along the way. Definitely get a balance if you feel a vibration, change your rims or have a flat repaired. Putting off a needed balance job leads to excessive tire wear, wear to your shocks, struts, steering and suspension parts. A balance not only improves your ride and handling, but also can save you some big repair bills and possibly an accident. Additionally, you will get better gas mileage.