Arun Isaac makes a nice point. Most of us support open source and insist on our right to have source code available but we hardly ever look at that source. Except for Emacs. Isaac observes that when the documentation is missing or ambiguous, he simply follows the link to the source to see what’s really going on.
Of course, you have to know some Elisp but the point is that the source is instantly available without leaving Emacs. You can check it and then return to your original buffer with your new knowledge. It’s life changing in a way that even the best of most open source software is not.
As I’ve said before, it’s because Emacs (sort of) recapitulates the Lisp Machines of yore. There are no secrets and everything is available in your chosen environment. This, more than anything, is why I am moving as much as possible into Emacs.