The number of revolutions, the characteristics of the road, the driving or turning direction, among other factors will determine the degree and location of worn areas in each tire.
Tire rotation is aimed to move the tires and wheels, changing their position, in order to even the tire wear and extend the lifespan of a set of tires.
Uneven wear of tires is caused by multiple factors. The different weight on the rear and front axles and the position of the engine (generally in front) are the most common. Front-wheel drive cars will also have the transaxle in the front, and this is additional weight over the front tires.
This is why front-wheel drive vehicles will need tire rotation more frequently. Uneven wear can also be a sign of mechanical problems, so ask your trusted tire repair service to inspect your vehicle for further malfunctions.