Initial ideas are,in most cases, the root to all leading developments and final ideas. This is why it is vital that in some form of way, either scribbled or constructively sketched, they are put onto paper for future use and revisions. To draw is to think, to sketch is to create. Sketching leads to physical product creation.
Wether the idea has been drawn in pen,pencil, chalk or even coffee stains, the idea is not only a visual concept, it displays the raw thinking process and initial idea that came into the head, expressing what your first intent of the design was, to which can be built upon and developed.
Sketching allows your creativity to expand and broadens your visions and imagination. What someone may visualize in their head could be completely different to how someone else sees it in their own mind. Which is why it is vital to illustrate the idea physically so everybody can view, process and understand the idea in the same way as the creator visualized it.
Have you ever had a concept in your head yourself, but no way to illustrate it?