Visual representation of your idea speaks a 1000 words and craves further creativity. Fastest way to present something, to brainstorm, and to come up with the concept is sketching by hand. It allows to think fast, progress fast... It cannot be done prior to milestones 1 through 4, as they feed the foundation for creating the concept. These are the first visible results that everyone can understand and discuss. It allows quick iteration and advancement based on solid background.