They are either for looks or if you are running a wheel/tire combo that requires them. If you DO get them you HAVE to get hubcentric spacers. If they are not hubcentric, you will get vibrations. You also need longer studs if you are going anymore than a 1/8" spacer so that means new studs or get the spacers that bolt on with their own set of studs. Again, they have to be hubcentric and the $50 ones on AM are not if those are the ones you are looking at.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.