Artists like that charcoal is black. When the grayness of graphite just doesn't cut it, charcoal can provide the contrast an artist seeks.
Artists' charcoal comes in a few basic forms. Vine and willow are the most pure, basically a burnt twig without any sort of additive or binder, which is why the sticks are delicate (the marks they make are delicate too).
Compressed charcoal is smooth and dense as it contains binders both to hold the stick together and to hold the marks it makes onto the surface of the paper. Charcoal pencils are essentially thin compressed sticks encased in wood.