The Markdown authoring landscape is very fragmented. Everyone seems to be doing their own thing, and reinventing things like content reuse and conditional text.
I think this is specifically meant for GitHub and showcasing code examples within readme files.
The idea is that you can show some code snippet within your project without having to update the readme file whenever the code is altered.
Seems like a cool idea, although I don't think this is necessary unless one has a very big readme file with a lot of examples.