Rogan started training BJJ in 1996 and has a background in kickboxing. He didn't even get paid when he started working for the UFC in the dark era, he was just doing it because he loved the sport.
He started working for the UFC in 1997 and started doing color commentary in 2002. Mix his background in Taekwondo with his 6 years of BJJ experience with his background in acting and stand up comedy and you have a guy that has real world experience in both grappling, striking, and public speaking.
He was also well known from one of the most popular shows (Fear Factor) on television at that time. If any thing the UFC was lucky to have him as they were struggling to be taken seriously. Rogan being a part of the team gave them some much needed legitimacy.