If you have directional tread on the tires, you probably don't want to swap them from one side of the car to the other. If you do, your tread will be facing the wrong way.
On many tires it doesn't matter, but some tires (especially high-performance and Aqua-tred type tires, the tires are only supposed to rotate a certain direction. You can probably look on the sidewall of the tire for an arrow marked "Rotation direction" or something like that.
Of course, you could always remove the tires from the wheels and re-mount them facing the other way, but that seems like a lot of work
Just rotate front-to-back if you have unidirectional treaded tires.