By rotating your tires, you give the tires a chance to even out their wear and get extended life out of your tires.
Without regular rotations, tire treads can wear down unevenly to create a rough and potentially unstable driving surface. In the end, this type of tire tread wear may decrease your safety on the road - think heat buildup, hydroplaning, poor traction in snow and ice, and an increased risk of punctures and blowouts.
Tires should be rotated every six months or 6,000 to 8,000 kilometers, whichever comes first. A good way to remember is, get your tires rotated when you get your oil changed.