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.

Because it won't be there the next time I go looking for it.

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.