You never know where a 20-second sketch will take you. And you can never make enough sketches. They are GOLD to flip through for developing future compositions, remembering places, and feeling good about all those times you picked up a tool and made marks in your sketchbook.
While attending a group event at an instructor’s studio earlier this year, I discovered her cache of sketchbooks—a TREASURE TROVE chronicling her moods, surroundings, experiments… It almost felt like I was violating her privacy, but she assured me she left them out for that very reason. I would have sat there all night turning her pages, but for the guilt of appearing unsocial. Back to the party I went, eventually.
Where will a preliminary sketch take you? ANY WHERE! Pick a favorite simple drawing, then use other media to draw/paint it again and see what happens. You’ll get something quite different every time.