This was the best pick Belichick made in my opinion.
I've wanted a big press corner in this defense for years, a guy similar to Sean Smith. A guy that could matchup with opposing big/tall WR's and all those "move"/receiving TE's that have given us trouble over the years. There were two guys I wanted: Dowling and Jimmy Smith. I preferred Dowling due to his physicality and tackling ability.
We've gone far too long with 5'9 or 5'10 smurfs like Hobbs, Wheatley, Hobbs, Butler, etc who all have struggled with bigger receivers (remember what Braylon Edwards did to Darius Butler in Week 2 of 2010?)
This guy gives you a tall, physical corner who can play in press vs those bigger WR's (Andre Johnson, Larry Fitzgerald, Brandon Marhsall, Vincent Jackson, Calvin Johnson, etc). He can also compete better vs those guys on jump balls, and he won't allow them easy releases like guys like Butler and Wilhite did.
He also gives you the versatility to put on those "move' TE's who are used heavily in the receiving game. This guy is big enough, fast enough, and physical enough to cover guys like Dallas Clark, Antonio Gates, etc.
Take Aaron Hernandez for example. If you're an opposing team, how do you choose to cover him? He's too fast for LB's. He's too fast for most safeties. He's too big/strong for most slot corners. Put a big CB like Dowling on him that can match his size and speed, and be physical with him at the line.