Having an idea is just not enough. You need to make that idea happen. You need to pick the right idea from 100 ideas, shape it and develop it.
As a designer and a creative professional, you have lots of ideas. You need to take those ideas to the next level. In this case, the next level is to put your ideas on paper. So you need to elaborate sketches. This doesn’t mean that you must have really good drawing skills – you will create rough sketches, which are quick and cheap.