Encrypted notification generally uses an algorithm based on public key or its derivative thereof. To facilitate this, the EPNS protocol maintains a public key registry of all the users who enter the ecosystem. This is done by having mirrored function with public key registry for creating a channel and subscribe which allows the protocol to emit public key of the wallet out (or skip if it already exists).
The public key registry for users is completely optional and the protocol has alternative methods that completely skip this process but carry the required functionality. The registry is helpful on the frontend side to quickly poll and find the key of the wallet and use it to encrypt the notifications wherever necessary.