With the ability to overlay carbon sheets and the ability to choose between varying levels of stiffness bike designers can create parts that are stiff in key places on a frame like the bottom bracket while using less, or lower grade fibers to reduce weight or increase the give in a frame. This is really what sets carbon bikes apart from other types of materials. Designers have complete control of the stiffness of a material in different directions at every part of the frame.
What Can Stiffness do?
A properly designed carbon frame is amazing because the bike doesn't have to behave like a uniform material. High stiffness carbon will make the bike fly when you stomp on the pedals and it will react quickly and decisively in corners. Manufacturers can take advantage of this in key areas of the frame such as the down tube and bottom bracket. But the fact that designers can choose to use less stiff fibers in other areas of the bike means that they can reduce the vibrations and impacts that occur when riding. Making the bike more comfortable as well.
And that's what makes Carbon fiber shine over any other material. It can be made comfortable, without compromising any stiffness or weight.