One reason I can think off the top of my head is archiving: Nothing prevents the owner of a repo from simply deleting it.

I do this. I have a ln instance of gitea running internally that mirrors any repo I have on github. Super nice for archiving things of importance or even as a bookmark. Sometimes I do it because of fear of censorship like dcma and stuff for software use.

Would you mind to share how to copy your setup? In particular how to mirror all your GitHub repos, do you have to manually add them to Gitea one by one?

I’d recommend Forgejo rather than Gitea for a new install these days. It’s a Gitea fork that was soft-forked (still compatible) until recently but is now moving to a hard fork model and has significantly more development momentum and a bigger community behind it. Still basically the same thing for most purposes, but I think Forgejo’s approach to actions/runners makes way more sense and they’ve started adding features like ActivityPub federation that I think will put them in a good position in the future.