You want to read the documentation or delve into the implementation of a feature of your programs it is literally right there in your editor instead of having to jump to some, potentially outdated, website documentation or having to figure out where to look at things from a sterile github page.
I wish that the idea of self documenation was more widespread in other ecosystems.
E.g. the ecosystem I know most, JavaScript, would have this potential, but very often dependencies are compiled and there is no such self-documenting culture so best case scenario you get types you can _almost_ trust and some jsdoc that you can _almost_ trust.
I guess the fact that the lisp and elisp community in particular has always cultivated this practice is the deciding factor rather than the languages itself.