Once you have a light bike riding anything heavier just feels wrong. Every bit of weight on the bike is added resistance your body has to overcome. The less the weight the faster you can make the bike accelerate. Its not just about forward motion either. Light bikes handle much better and react to the riders movement more quickly.
Carbon fiber offers impressive strength to weight ratio thanks to the strong directional properties of carbon chains. Because the fibers are only strong directionally sheets of the fibers are often overlayed at different angles to provide better all around structural preformance. This allows bike manufacturers to build frames to be light yet still strong enough to transport a rider.
How It's Made
Carbon fiber is produced from polyacrylonitrile or (PAN). This polymer is heated above 1000 degrees in an inert atmoshphere to bake off all of the non carbon constituents in PAN .This leaves behind pure carbon strands. The more the PAN is baked the higher the elastic modulus of the fibers can be. Once the fibers have been extracted the shape is generally molded and then a resin is applied to the fibers in order to hold them together. The epoxy also has the added benefit of decreasing the brittleness of the material.
Its Not Uniform
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.