Also:
1) There are use cases (protocol statistics or scientific/historical archives, for instance) that require the full data set.
2) The NSA relay and the Kreml relay will never hard-delete.
3) You may specifically want to write to a relay that promises to maintain the original event, if you are worried that someone might force you to publish a delete event. This would be a sort of WikiLeaks-style relay.
And
4) You may want to keep the original event available for yourself or a community, in an archive relay, but hide it on everyone else's relays and signal that you're fine with it being deleted.
Similar to keeping multiple versions of replaceable events. Most relays will just delete them, but there is a case for a historical relay tracking versions.
And
5) Some relays are linked to blockchains and they can't hard-delete a chain, by design.
Like this one, from nostr:npub1fjqqy4a93z5zsjwsfxqhc2764kvykfdyttvldkkkdera8dr78vhsmmleku
