Hiya,
First off i lay no claims to this concept..but i have witness alot of what i would consider the top drawers on this site using a form of this concept.This does not only apply to traditional forms of art such as graphite, color pencils,painting but applies to just about all of the mediums and includes photography.
Shading:For realistic drawing it is a must,but if applied wrong this can make your drawing flat , with no depth.done right you will achive a level of drawing or painting etc which will draw the viewer in with no effort.
Study: Most of us use a Ref photo, study it before you start drawing it,plan your drawing out
and keep to