Currently, if a repo is on github, and you develop a patch on a fork on gitlab, you can't just click a button to open a pr, the way you could of both upstream and fork were using the same provider. Federated PRs would be the capability for one provider to "talk" to another to be able to pull your patch from the other provider.

Being able to host software on my own instance, without requiring everybody else to sign up there to contribute.

I like to self-host, but I don't want to be sysadmin to a public service, and deal with account deletion, password resets, ...