Traditional artists have to do thumbnail sketches and pre sketches before the actual final composition sketch.
its usually like a 10 - 15 drawing process so we draw the subject matter several times over. The problem is the fact of layering and how the materials attach to the paper. When you layer even if you erase your slowly flattening the tooth of the paper making it less effective to draw on, sometimes your even applying layers of whatever medium you apply to the point you cant erase anymore so you have to redraw from a new sheet of paper.
Where as in digital i figure you can just white out a part and redraw it without having to worry about these issues.
Some artists who do compoisitions out of imagination spend more time doing sketches and pre-drawings before they finally decide their final composition for their final piece.