As an open source project, Atom encourages random people to mess around with it and use its functions. Obviously, anybody can sift through the source code and pull out every callable function, some of which might be legacy code, but then there are some packages that actively use these undocumented functions. Packages, even core ones, are separate from the base program. When people like us try to figure out how to write new things for this editor we like, we’re going to look first at packages that do roughly similar things.
I believe that I’m on the same page as @UziTech in believing that, when a function is used in a package but doesn’t have any information available on it outside of the source code, that’s an issue. If it’s on a list of methods that are works in progress, deprecated, or otherwise prone to change in the foreseeable future, then that’s useful information for us randos. Instead of talking about mysterious undocumented functions, we can talk about whether or not we believe a particular function merits inclusion in the public API. If a member of the community stumbles upon a function that they love, but the devs believe that it’s not ready for whatever reason, the community member could work to make it ready. That’s something that can’t happen if this information isn’t readily available and we have to have this discussion about documentation every time.