Not the OP, but I have needed to patch Qt due to bugs that couldn't be easily worked around.

I have also been frustrated while trying to interoperate with expensive proprietary software because documentation was lacking, and the source code was unavailable.

In one instance, a proprietary software had the source code "exposed", which helped me work around its bugs and use it properly (also poorly documented).

There are of course other advantages of having that transparancy, like being able to independently audit the code for vulnerabilities or unacceptable "features", and fix those.

Open source is oftentimes a prerequisite for us to be able to control our software.

It has helped prevent problems. I am not worried about a python suddenly adding a clause stating that I can’t release a ML framework…

In the earlier days of rustc, it was handy to be able to look at the context for a specific compiler error (this is before the error reporting it is now known for). Using that, I was able to diagnose what was wrong with my code and adjust it accordingly.