To the left are two cylinders. The one on top is drawing using what I call "contour lines" The bottom cylinder uses random cross-hatching lines. Both are used to give the cylinder a more 3-D and dimensional look. Which one do you think is more successful?

While both cylinder shapes look...well, cylindrical, I believe the top cylinder looks more 3-D. That's because the "contour lines" go around the cylinder, which emphasizes its roundness.

The bottom drawing gives the cylinder a shadow, but it doesn't have as much of a "3-D" look. The lines do nothing but suggest a tone. They can suggest dark, less dark, medium, light - that's all they can do. Actually, if you look at the bottom cylinder in a certain way, you aren't sure if the rendering is really supposed to be a shadow, or just a dark stripe down the side.

I personally find myself using a certain amount of "dimensional" or "contour" lines in my drawings, paintings - anything I create. I can feel the depth and shape of the thing I am rendering, and some of my brush strokes show that. Other brush strokes (or pencil lines, etc.) are just used to indicate tone, to deepen the shadow. But there are always "contour lines" in there somewhere, helping to give that suggestion of depth.

This is not to say that all drawings must use contour lines. Some artists have fantastic rendering techniqes which don't allow them. Like - painting everything in little dots, random hatches, and so forth. No one technique is suitable for every artist.

But, when drawing faces, I find that these contour lines to be so alive. So I prefer to use them.