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Warnier diagrams are not well known. They were introduced in the age of Structured Systems Analysis and are now forgotten. However they are the best tool for solving complex problems. Sometimes you need to model complex logic with multiple different exception scenarios. Sometimes you need to come up with requirements for complex functionality like sorting elements by multiple parameters or importing/exporting data from excel to system database.

There is an excellent book explaining how to create Warnier diagrams: "Structured Systems Development' by Kenneth T. Orr. You need to read this book to learn how to creare this type of diagrams. But it is well worth doing.  This is quite short read with multiple diagrams and very well explained. One you start reading it is hard to stop until you get to the last page. Below is an example of one diagram I made to gave you a feeling what this is all about: