Carbon allows them to build super stiff bikes at a reasonable weight. It might only take a pound or 2 off the weight of a bike, but since they can use bigger tubes, monocoque designs, and optimize the carbon layup they can make the frames far stiffer than any metal bike. A stiffer frame enables the bike to hold more aggressive & faster lines and lets the suspension work better. And if one of the factory pros breaks a frame they just pull out a new one from the truck.