Personalised relay feed for pubkey, based on a web portal or similar - like filter events with ‘Facebook’ before sending to me. Or Nthdegree events only.

Access premium features that are too costly to provide to everyone for free - search, POW generation, extra metadata on events, perhaps media caching, etc.

Unlocking higher rate limiting. For example re-broadcasting a backup.

Perhaps access to your recently deleted events.

I’ll keep thinking. It does have a privacy consideration which isn’t ideal - but can’t see how you could get past that.