This flag is used by some projects like https://github.com/status-im/nim-faststreamsto offer a way to pick between chronos and asyncdispatch - by using a "standardised"name for it, it becomes easier to migrate an entire stack of libraries, which is why it's documented in the std lib (even if std lib doesn't use it on its own).
Thank you for your response. I think one of the core concepts of backend is dispatcher and scheduler. I was curious about how they work and if it's possible to replace them with the ones from the std lib. I tried to search for information about it, but I couldn't find a starting point, so I asked this question. It seems that the ones inside the std lib are only using asyncdispatch.