Systems Which Feed Themselves
A system which feeds itself is a system which contains, within it, the tools to grow. It's like a self replicating machine.
The example I've used in the social sciences is the subject of history - it feels, sometimes, like it exists solely to produce other history teachers.
And the use of Emacs, in the end people spend so much time configuring it rather than actually using it for anything "outside".
For some programmers, nothing beats writing tools for other programmers.
A good chunk of my blog is about blogging.
A good chuck of people's Zettelkasten are about maintaining and using their Zettelkasten.