Well, the M3 GT3 isn't just a track car, it could be a daily driver too....just a badass one. It's 500lbs lighter than the regular m3, has a bored out engine that produces 50 more hp and has tuned suspension for better handling.....BMW can't just hand the trophy to porsche....even if the porsche is better overall, it still needs competition.
But on a serious note, the GT3 was made for niche BMW fans, its for people who want an m3 for performance, not just to say you have an M3 (even though the normal m3 is fuckin beast). BMW's and porches drive a lot different and some just care for BMW's style more than porches....