The move could essentially be described as trading from a strength to address a weakness, as the folks over at Camden Chat note:
The bullpen is a place where the Orioles have some surplus. With Jim Johnson, Pedro Strop and even Darren O'Day turning in great seasons, the back end is pretty well locked down. Trade the seemingly least-needed piece of the 'pen for something you do need - a starter - and that's probably about what you want to do.
When it comes to Saunders, his biggest value comes from the simple fact that he's a starter. He generally doesn't put up stellar ERA figures and he's been homer-prone at times in his career, but he eats innings with efficiency and rarely gets hurt. This season, he's posted a 4.22 ERA over 130 innings, pretty standard numbers for the 31-year-old.
Going forward, this deal isn't likely to have major implications for either team, but it does allow Baltimore to add a useful piece that wasn't exactly necessary in Arizona. For the price of a relief pitcher on a one-year deal plus a 2013 club option, it's a pretty reasonable deal for a contender.