It's encrypted
Discussion
I know. That's generally how people put private information on relays. Relays are not only for public data.
And don't forget private relays.
What’s the difference between a local private relay and a local database?
A database in a server doesn't store signed events and can be changed by the server at any time.
Users that use private relays are generally the controllers of such relays and or have ways to decide which apps get access to them and how to move the data to another private server.
It's about allowing the user to decentralize from that specific app by simply moving events between relays.