You lose Cortland Finnegan in free agency and replace him with a first round cornerback. Kirkpatrick has played some of the best competition and won a lot of games, so there's reason to believe he will do a fine job replacing Finnegan.
SB Nation's Titans blog, Music City Miracles, explains the pick:
The Titans need help at almost every spot on defense. Most people believe they will take the best defensive player available, and Dre Kirkpatrick is just that in this scenario. He has top 10 talent, but has been sliding down the draft because of his arrest earlier this offseason. Kirpatrick's weakness is that he isn't a very good man coverage guy, but the Titans don't play very much man. He would be a perfect fit for their system. Adding him would allow them to keep Alterraun Verner in the slot where he is best.