Wheel spacers can be used for a variety of reasons. Spacers mount between the hub and the wheel, locating the wheel further from the suspension.
Wheel spacers widen the track of a vehicle, and can increase grip during cornering. It should be noted that wheel spacers can also result in heavier steering.
Another reason that wheel spacers are used is to improve the appearance of a car. By pushing the wheels further away from the body, a better stance (the way the car sits) and more aggressive look is achieved.
Wheel spacers as wheel accessories might also be used to provide additional clearance between the wheel and the suspension or bodywork. For instance, if wider tires are installed, and one rubs on a suspension piece without a wheel spacer, adding a spacer may solve the problem by locating the wheel further away from the part that it is coming into contact with.