Private relays make it possible to de-couple the data layer from the interface for private apps

Example: if you use bitwarden, you are stuck with the bitwarden API, but maybe you like the lastpass interface better 

But if your passwords are stored on your private relay, you can swap between any interface you like without having to do imports/exports and installing new things

Basically static, app specific APIs can become obsolete with a good set of NIPs