The Panthers had some defensive tackle trouble last season so Cox fills an immediate need, which is to help shore up the run defense. With Charles Johnson at defensive end, the team needs to establish a strong interior presence. SB Nation's Panthers blog, Cat Scratch Reader, explains the pick:
There is no question the Panthers will look to bolster the defense with this draft, and that starts up front at DT where the Panthers were terrible at stopping the run last year. Though the Panthers would prefer an elite pass rusher like Quinton Coples, with Coples off the board, instead they take Fletcher Cox as the best DT in the draft. In spite of Dontari Poe getting all the hype from the combine Cox was just as impressive and is a better fit for the Panthers scheme.
Cox can play anywhere along the 4-3 front line, plays with a relentless motor and is the best DT in this draft getting off his blocks and making plays against the run and pass. Cox played against top flight competition and is a strong, agile and athlete DT who can make plays in the backfield and might end up the best interior pass rusher in this draft. Another big plus is no one questions his effort and he enters the NFL as a high character guy with no red flags.
Fletcher Cox 2012 NFL Draft Profile (via sbnnfl)