When you need documentation that has to last a _long_ time and be editable by multiple people. Technical diagrams that can be revised and change tracked an item at a time are priceless in those scenarios, whereas you will never know if draw.io will be around in a few years' time.

> you will never know if draw.io will be around in a few years' time.

https://github.com/jgraph/drawio#readme (Apache 2, at least for now)

Its creators use it for "railroad" syntax diagrams in the SQLite documentation. For example: https://sqlite.org/lang_select.html

What if you want to have version changes to the diagrams tracked by git?

I maintain a Markdown-based website. I considered using diagrams to explain certain things. It would be a pain to keep image files and rebuild and export images every time the diagram changes.

Instead I can put the diagram code directly in the Markdown file and update it as needed. Version control would track those changes legibly.

This is very similar to using inlined LaTeX formulas instead of screenshots of the final mathematical formula.