Rowing's benefits are very similar to running except much better. Rowing effects your cardiovascular system. Depending on the intensity of the rowing it can target both anearobic and aerobic systems. It's also very effective at getting your posterior into the action. The real key to what makes rowing such an effective conditioning tool is that it is a multi-joint functional movement that works both the hip and shoulder joints. This is much more effective then say just running which has very little upperbody involvement.
Rowing is also very conveniant. In many cases some combination of exercise are hard to perform if you dont have a track right next to your workout center. Rowers are on the spot.
Hope that helps=)