Universities might want to locally host a mirror in order to waste less bandwidth and provide 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;
The question here isn’t “why would you” but rather “why would I be unable to do it”. Their actively gatekeeping their repository in a futile attempt to be the single and central point of flatpak distribution - much like what Apple does with the App Store.