Note that many of Edwards' assumptions about "handedness" in brain structures and specific conclusions about how drawing affects creativity have been challenged.  Think about her general thesis, that drawing engages a different set of cognitive systems than verbal composition.  To the best of my knowledge, nobody has successfully disproved that.  Using drawings, diagrams, and sketches might offer writers alternative ways to understand what they are writing about, to whom they are writing it, and what they wish to achieve.