Tires rotate all the time as they roll down the road. But that's not what it means to "have your tires rotated". Rotating tires means moving them from one wheel to another and from left to right or back to front, or even diagonally.
Why rotate tires? Tires tend to wear unevenly. The front tires tend to wear on the outside edges because the tire leans over when you turn a corner. The rear tires just follow the fronts, so they usually wear more evenly. If you leave tires in place, those outside edges on the front tires will wear down much faster than the rest. You'll have to replace the tires thousands of miles sooner.
A poor alignment or unbalanced tires can also cause uneven wear.
Rotating tires keeps them from wearing unevenly. That will make your ride smoother and handling safer. And it extends the life of your tires, saving you time and money in the long run.
There are 3 key things that should happen during a tire rotation: