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I'm a CS student, currently a junior, and in one of my classes this semester they have us studying all sorts of UML diagramming methods. Among others, we've touched on Petri nets, DFD diagrams, sequence diagrams, use case diagrams, collaboration diagrams, Jackson System Development diagrams, entity-relation diagrams, and more. I've worked on more than a few professional projects over the years and never encountered anyone who used these systems to any great degree (other than a general class diagram or a diagram of the tables in a database). I was just wondering if I could query the hive mind to see if this is true in your work experience too.

Have you used these models at all and found them to be as important as they tell us students they are? Or is all this stuff just academic ivory-tower crap that people in the real world hardly ever touch? Which of these systems have you found to be effective and useful? Are there specific kinds of scenarios that they are more intended to be used in than what the typical software developer encounters?