Well, for one it has mixed wall potential, as it has 120 HP - 120 Def - 135 Sp Def as base stats. Real mixed wall potential, not like Bold 252 Def/HP Blissey. It works great with Calm Mind for sweeping and tanking. It takes 40-30% from Choice Banded Tyranitar's Crunch, and can KO with Grass Knot. It takes around 25% from SpecsMence's Draco Meteor, and can KO with Ice Beam. It is just about the only thing that can stand up to BellyZard without a Focus Sash (which, despite making it a counter to an obscure threat, really tells you something about its walling capabilities - special and physical). Complete Ground immunity without having a Rock weakness is a handy thing to have indeed.
Basically, Cressy rocks.