As ubiquitous as diagrams are, they have some significant shortcomings.
Diagrams tend to be most useful as a mental organizing activity for the people who make the diagram. Outsiders coming in fresh have a much harder time seeing how the elements of the diagram fit together and make sense. I think that is generally because diagrams lack enough narrative to explain what is going on. We often use diagrams without explaining them. Or, more precisely, we use diagrams without connecting the diagram pieces to their associated parts of our narrative.
Read on for an example.