The tires on your car do different jobs depending on where they sit. On most vehicles the front tires handle steering and carry more of the engine's weight, so they wear faster - and often unevenly across the tread.
Rotation moves each tire to a new position on a pattern suited to your vehicle's drivetrain. The wear that has already started gets spread across the whole set rather than concentrating on two tires.
It is one of the cheapest things you can do for your tires, and one of the few that reliably adds miles to their life.