Because the stereo system in my Mustang doesn't have a USB port, and I can't afford a reallllllyyyyy long USB cable.
So, just so I have this right, MP3 files burn to CD just like WAV files, without any additional software required?
Well no. But the needed software to go from MP3 to CD format is part of every CD burning package I've ever seen. Both iTunes and Windows Media Player have it, for example.
What year is your Mustang? Many of the newer (post '05 or so)factory stereos will read MP3 format which vastly increases your song/disc ratio.
Lastly, if budget allows, I strongly suggest ditching the discs. I put a Pioneer Nav unit in my Cobra when the factory Mach system crapped. I got the head unit, USP/iPod cable, mounting kit, and Ford specific wireing harness for about $500 delivered. Some solder in my garage, and 35 min in the drive way, and I have WAY better sound quality, touch screen, GPS, and the abilty to plug my 160Gb iPod in (inside the center console) and have more music then I could want. And no fumbling with discs, reburning scratche CDR's (they seem to scratch easier) or having my passengers step on CD cases. Best car audio move I ever made.