Design is directing the eye. When you are drawing (especially at the sketching stage) you are directing the attention of the viewer with line. That's why it's called disegno.
Quick sketching is basically making notes on where you want shapes and weight to go. You do this by way of exaggerating certain lines instead of copying the them from the form. That's how people develop a style. It's simplification. It's your understanding of the structure of your object, which deepens the more you study it, until eventually your shorthand come close to being as subtle as life itself.
I don't know if the line drawings are actually worth posting at this stage. I'm not very good at drawing on my tablet.