These sketches are not moments of genius and they certainly didn't appear out of nowhere. Instead they are the result of hours of toiling away on the drawing board.
We love to sketch on paper because the act of drawing forces you to make decisions that are too easy to ignore when drawing on the computer.
A simple sketch is valuable in a couple of ways. Sometimes at the start of a project, the idea represented in a sketch is so strong that it can help form the bones of a design. Other times, when a project becomes bogged down in details, a sketch can remind of where you came from and can orientate you toward what you are trying to achieve.