Just as the saying stands: "Reduce everything to its essence so that form harmonizes with function", graphical diagramming notations have considerably helped for better communication and understanding in Software Development.
A diagram can help us better express our ideas about Software Design without putting textual explanations. It's already confirmed that human brain better processes images than texts. That's why graphical representation through diagrams is quite effective to communicate and express our ideas either for a Software System or a Business Process.