Pictures communicate what words can't.

Drawing is a good way to develop an understanding of how something works. One can communicate not only what exists, but also what could be. 

Words and sentences must, by necessity, come only one at a time in a linear logical order. Systems happen all at once. Their elements are connected not just in one direction, but also in many directions simultaneously. Pictures work for this language better than words, because you can see all the parts of a picture at once.
— Donella Meadows, Thinking in Systems

A picture enables people to take in the whole. As designers, this is important as we we work to make sense of infeasible parts.