Signing and encrypting are different operations. The assumption here is that all devices can sign with the same key, but not encrypt.
Discussion
> Signing and encrypting are different operations
Perhaps in the diagram you could include the premise that the user is operating in the two clients with a signer that cannot encrypt (FROST bunker); this would make it easier to understand why such a structure becomes necessary.
Even if there are probably other uses cases, like
This image was just thrown out of context for no reason, I didn't expect it to be shared so much.
Anyone interested should read the NIP proposal I linked above (too late).