Inspiration and a clear vision are the starting points for creative tasks. It's on a blank sheet of paper that they first acquire form, solely through a pen and the hand that guides it.
The sketch is the most emotional form of an idea. This is why the designers first work out their ideas exclusively with a pen and paper. The concern here is not feasibility. Instead, the sketch is intended to bring life to the character of the model, which then remains the guiding image throughout the design process.