From the folks at the Baltimore Orioles:
Incentives for fans to "check-in" using At the Ballpark include many new promotions for the 20th Anniversary season of Camden Yards. First, an "I'm Checked In" T-shirt for the first 20 fans who check in and visit Fan Assistance behind home plate each game to claim their prize. Second, the 20th fan to check in each game will unlock an opportunity to upgrade their seats to a Field Box location. Finally, the first 20 fans who check in to at least 20 games at Oriole Park using the application by September 13 will unlock an offer for the opportunity to watch batting practice from the field at a future game.
More information on MLB.com At the Ballpark is available at www.orioles.com/checkin.