It's used as a guide, and helps ensure that the model stays in proportion. Often a front, side, and top view of a character is drawn, then these images are added either as backdrop images or textured plains. Then, it's a lot easier to make the character model using them as a guide, usually on a vertex by vertex basis.
It's a good habit to get into, it can help with everything from planning the texture maps to allowing enough vertices for smooth animation. Character modelling is one of those things that can be made a lot less painful with some extra preperation.
I got a fever, and the only prescription, is more memes.
