One of the number one reasons wheel spacers are so popular is cost. You get a surprising amount perks without having to shell out for an entire new set of rims with different spacing as your old ones. So what are these perks, and when do fellow Mustang owners use spacers?
First up is aesthetics. Pushing out your stock rims so the rims are in line with your Mustang's wheel arches gives your ride a more streamlined and aggressive look while drawing attention to your rims. If you already have a set of aftermarket rims (wider or not), wheel spacers can also correct backspacing issues so your aftermarket rims look their best. Wheel spacers can also make it so you can fit wider rims under the front of your Mustang without them turning and colliding into suspension components. You can even combine spacers with aftermarket suspension components to further increase your steering angle!
Further on the performance side of things wheel spacers are more for brake clearance than anything else. The beefier calipers in big brake kits don't always fit under the stock rims. Pushing the rims out an inch or so will give the calipers more room to breathe, and you don't have to worry about scarring the inside of your rims or the brand new calipers.
There are some things to keep in mind before slapping on a set of spacers and calling it a day. Too big of a spacer can actually hurt your Mustang more than the wider stance could help performance. Oversized spacers can negatively affect the scrub radius (imagine two lines: one through the center of the tires and another following the angle of the suspension. The difference between where those two lines meet and the road surface is the scrub radius). Ideally your scrub radius will be zero for optimal performance. Too large of spacers can move the scrub radius and overstrain suspension components (wheel bearings, ball joints, and tie rods), making them wear out faster. All considered, make sure you don't over shoot what you need.