Just as oil needs to be changed periodically in order to prevent engine damage, and just as the car's exterior needs to be washed and waxed periodically in order to preserve the quality of the finish, tires need to be balanced--at least at the time of purchase.
In a perfect world, perhaps tires and wheels would be made with such precision that they did not need balancing, but in our imperfect world, tires do need to be balanced--if you want to have a smooth ride and smooth handling, if you want the tire tread to last as long as possible, and if you want to prevent some types of suspension/steering damage.
And, in case nobody mentioned it to you, the inflation pressure of those tires should be checked at least once a month, and the tires should be rotated every 5k to 10k miles (consult your maintenance schedule for details). Failure to take care of inflation and rotation will drastically shorten the tread life of the tires, and can also result in poor handling and reduced fuel economy.
All of this is just one small part of the maintenance of your car.