Just participated in a very useful SCBWI webinar with author/illustrator Arree Chung. Arree was focussing on the process and reason for creating Dummy Books in developing picture books. He finds the process of creating 'dummies' enables him to refine and develop his initial story ideas and as he says, 'turn ideas into something real'.
A 'dummy' narrows your focus and helps you to see problems earlier, in either your manuscript or your illustrations or both. Arree recommended the 'creating a dummy process' even for writers without illustration skills, and encouraged everyone to have a go.
It was fascinating to see his process through developing his recent picture book 'Mixed' and how important the dummy book was in that creation.