I use drawio, as those diagrams are quite future proof (drawio is open source, very old and you can embed diagrams in svg files) but also very expressive (you can even use latex inside drawio diagrams).

Since I created a drawio integration extension for VS Code, I tend to create much more diagrams during coding, as they are really cheap to create. There is also an extension for IntelliJ and the diagrams work nicely in github readme files. After moving diagrams closer to code, I noticed that it is much easier to keep them in sync with the code base. Especially as PRs changing the code structure can include diagram changes at the same time.

I use them to model simplified class diagrams for documentation purposes, to model data flows, to create primitive mockups, to show the relationship between UI components (as embedded screenshots) and to show component dependencies. I try to follow UML, but I don't care about machine readability and prefer readability over UML compliance.

You can find some uses-cases of those diagrams here (https://dev.to/hediet/create-diagrams-in-vs-code-with-draw-i...).