Universities might want to locally host a mirror in order to waste less bandwidth and provider faster downloads;
Large companies usually like to host internal mirrors for the same reasons as above and also so they can audit and pick what packages will be available for their end users;
Yeah sure, just try to mirror Flathub into your repo.
I don’t get it. Why would you store all of it? I mean, you can but… why?
Because…
For what’s worth Debian archive repositories are about 5 TB and people actively mirror then in universities, companies, cloud providers etc.
You can but there would be little point. Fedora has its own repo for instance.