Traditionally, riders used the Giro to prepare for the Tour; especially back when they'd show up at the first of the season overweight and out of shape. The past few years have seen more and more Tour contenders using the Giro as part of their Tour tuneup. A 21 stage race is definitely different than the other races taking place at the same time as the Giro.
Not only is Contador the top gc contender, he's got the best team behind him.