Tire rotation or rotating tires is the the practice of moving automobile wheels and tires from one position on the car to another, to ensure even tire wear. Tire wear becomes uneven for any number of reasons. Even tire wear is necessary to maintain consistent performance in the vehicle and to extend the overall life of a set of tires.
By design, the weight on the front and rear axles of your car is different, which causes uneven wear. With most cars being front-engine cars, the front axle usually carries the majority of the weight. For rear wheel drive vehicles, the weight distribution between front and back is near 50:50. Front wheel drive vehicles also have the differential in front, adding to the weight, with a typical weight distribution of no better than 60:40. The result of this is that the front tires wear out at almost twice the rate of the rear tires, particularly when you factor in the included stress that braking adds to the front tires. Therefore, tire rotation for front-wheel drive vehicles is even more of a necessity.
Turning your car (which is unavoidable) also contributes to uneven wear. The outside, front tire is worn disproportionately. In right hand traffic countries the left front tire wears faster than the right front. Also, right turns are tighter than left turns, causing more tire wear. On the other hand the sidewalls on the right tire tend to be more often bumped and rubbed against the curb while parking the vehicle, causing asymmetric sidewall wear. As would be expected, the exact opposite occurs in countries that drive on the left hand side of the road.