Sometimes, I skip the ”illustration” phase. I think you should experience it several times to learn how to use it. However, it is a method to record relationships. I do it only if I want to record some relationships that I can not express only by the spatial arrangement of pieces.
For example, I draw arrows when two groups are conflicting with each other. I sometimes draw arrows to describe the flow of the story, especially the flow is not top-left to bottom-right. I seldom enclose the pieces of a group, because it is obvious to me.