Now, gel ink pen on aquarelle/watercolor paper, that's luxury.
I've mentioned before that on the pencil sketches and notes I make, the paper is more like cache for my mind. After I've found a solution/design, I make a cleaned up version (often on the computer, using Inkscape, Dia, or EasyEDA). (I don't waste the paper much, though: I often use whatever small area is still free of scribblings for smaller details.)
For similar reasons why the Rubber Duck method helps software developers, the sketching helps me design and understand algorithms and complex structures and interconnected systems.