When painting a subject in tones, you have dark, medium and light tones overall. But you have more than 3 shades! So what sense does it make to paint everything 3 tones? It leaves your painting flat and more of like something in the pages of a comic book. Also in different areas of the subject (even within the same area), there are variations of those tones. This is creating form and interest for the viewer, as well as for you as the artist.,
Now please don't take this personally if you do paint like this. I am not singling anyone out as I myself have done this, BUT if you wish to improve, please take heed. Take the time to paint with care, leave the 'paint for numbers' for others.