It's easier and enables more features to have 1 common platform.

For example, the social features of GitHub, which I like (like stars, browsing repositories by tags etc..)

But also For PRs, the way to make a pull request to a repo hosted at A, from your own node hosted at B.

And like other commenters said, you can do this workflow with git over email like a lot of projects to, but the main goal of the federation here to me is the user experience, the UI being able to link all of theses separate repositories, issues, PRs, etc, like everything was hosted at the same place.