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;
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.