The Bengals lost CB Johnathan Joseph in free agency last year and they need to replace him, which is where Gilmore comes in. He projects as the future starter and there seem to be few question marks about his game.
SB Nation's Cincy Jungle explains the pick:
Even though the Bengals have loaded up on cornerbacks in free agency by bringing back Adam Jones and signing Jason Allen, Cincinnati needs to address the future of the position and figure out how they're going to replace Johnathan Joseph, who left in free agency before the 2011 season. The only way to do that is in the draft and with the talent at the cornerback position in the 2012 draft class, the Bengals are lucking out. Gilmore is a talented, mature and team-oriented corner who could contribute right away and take over as a starting cornerback down the road opposite of Leon Hall. With Gilmore and Hall in the secondary, defensive coordinator Mike Zimmer will be able to be much more creative with the front seven when it comes to the pass rush.
Stephon Gilmore 2012 NFL Draft Profile (via sbnnfl)