The best tool for visual thinking.
"Visual thinking — and its sibling, visual communication — are foundational skills for interaction designers. Diagramming is the most commonly found tool in the designer’s arsenal. It would be difficult to design effective behavior without diagramming. We diagram to inventory and categorize the users we observe, the functionality we have to deliver, and the technical challenges we face. We diagram conceptual models of the way our users conceive of their work and how they understand their tools and tasks. And we diagram the course of work we have to perform as members of a team."
Article: Know Whiteboards, Know Design.