One can distribute flatpaks along with their dependencies on USB drives (or network shares, etc.) which is especially helpful in situations where Internet access is limited or non-existent.
Cache/mirroring would be great for those who need it.
Edit:
Thinking about it, I wonder if there's enough "core features" with 'create-usb' that its just matter of scripting something together to intercept requests, auto-create-usb what's being requested and then serve the package locally? If a whole mirror is required, it may be possible to iterate over all flathub packages and 'create-usb' the entire repo to have a local cache/mirror? Just thinking "out loud".
Thinking about it, I wonder if there’s enough “core features” with ‘create-usb’ that its just matter of scripting something together to intercept requests, auto-create-usb what’s being requested and then serve the package locally?
The issue is that... there aren't enough “core features”. It doesn't even handle different architectures and their dependencies correctly. It wasn't made to be mirrored, nor decentralized.
Apt for instance was designed in a much better way, it becomes trivial to mirror the entire thing or parts and for the end tool it doesn't even matter if the source is a server on the internet, a local machine, a flash drive or a local folder, all work the same.
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.