Wheel spacers alter the offset of the wheel; the distance between the mounting surface and the center of the rim's width (What is Wheel Offset? - Les Schwab). They may be necessary for aftermarket wheels that lack the OEM wheel's offset they are replacing. There may be times when a spacer can be used in conjunction with suspension mods to improve handling, but with factory suspension that's rarely if ever the case. Putting spacers on will give a wider track (which may look "better" to some), usually at the expense of proper handling and excess stress on the suspension.