These cars were a bit before my time, but when I was little I do remember the importance of white sidewalls on the tires.  If your tires were all black, you were slumming it.  If you had the white sidewalls, you were cruising in style.  On a muscle car, you needed some big tires with the white lettering on them-- and if the tires on the back wheels were bigger than the ones on the front to give you that dragster look, even better.