Stdout is about regular, processable output (as in reusable for further processing). It's usually the result of the command.
Any additional/exceptional output such as notices, errors, logs... should go to stderr.

Also, while the core command might not output anything useful to stdout, the command might be extended in userland. Hence we prefer outputting everything to stderr in that case.

I always considered stderr suitable only for errors / warning / exceptions. I am surprised but looks like Symfony's approach is quite common after more googling. I should have googled more before asking here. Sorry and thank you.