I'm genuinely asking; why do you want to download the whole archive? What's the purpose?

Preservation through proliferation. Should the archive be forced offline, for whatever reason, it will still persist elsewhere, even if its just rotting on somebody's external hard drive, waiting to be found.

AI training?

Can't speak for the above, but for me it's a hedge against enshittification and sudden shutdowns. If I had a dollar for every community archive that suddenly went offline, I'd have several dollars.