I think generation helps with content and structure that GitHub Markdown does not support, as well as having a dedicated (and brandable) website.

An example:

- source: https://github.com/eslint/eslint/blob/main/docs/src/rules/no...

- rendered: https://eslint.org/docs/latest/rules/no-control-regex

This is a very simple example, you can see how the code blocks are highlighted better and slightly interactive.