Starting with the 2011 draft class, contracts could be structured to give the team an option for the fifth-year, and so we've been seeing a lot of teams pick up those options recently. Castonzo is the 13th player from the 2011 first round draft class to have his option exercised.
The 22nd overall pick by the Colts in the 2011 draft out of Boston College, Castonzo was originally drafted to protect the blindside of the legendary Peyton Manning - who was getting older and dealing with some neck issues. Those neck issues turned out to be very serious and Manning missed the entire 2011 season. So, instead of protecting for perhaps the greatest player in league history, Castonzo was left protecting the blindsides of guys like Kerry Collins, Curtis Painter, and Dan Orlovsky. In 2012, however, the Colts drafted Andrew Luck with the first overall pick and ever since Castonzo has been the best and most consistent player on the offensive line, starting every game of the Andrew Luck era and 44 out of a possible 48 games in his NFL career (missing four games his rookie season but none since).
As I mentioned, Castonzo has been a very good player at left tackle for the Colts. He is likely the best player on the line and with so much poor play and shuffling going on with the interior offensive line last year, it is incredibly nice to have a guy who can play well and play ever game protecting the blindside of Andrew Luck. Left tackles are very important in the league, and the Colts continue to show that by picking up Castonzo's option.
This means that he is now under contract with the team through the 2015 season, easing the financial burden of next year's free agency significantly. Players like Reggie Wayne, Cory Redding, Hakeem Nicks, Darius Butler, Jerrell Freeman, Ahmad Bradshaw, Delano Howell, and others are set to become free agents next season (per spotrac), but not Castonzo anymore - who was likely going to get one of (if not the) biggest contracts out of that group. Of course, there will be other huge contracts coming up in the future too (like Andrew Luck), but at least it gives the Colts some more flexibility to figure out what to do next year.
There's really no downside to picking up the option other than the money, and Anthony Castonzo has earned it. This move comes as no surprise, but it's still noteworthy because the Colts left tackle is under contract for another season now.