I find very useful working with flow diagrams when in the middle of complex development systems and it’s absolutely a must-have when you are part of a team. Nobody can access your mind, so nobody can be sure about your thots and the internal flow diagrams running and changing continuously in your brain. So, a flow diagram with read/write access is a great feature for all developers working together in the same cake.
Appart from that, you will find many times, when working in the graph, that you are missing something you must implement, something you forgot when you were in UI-mode fighting against fonts and colors.
That’s the difference between conceptual and abstract design.