The user needs to scan a QR code with the nostr+connect value, and then keep the second device online at all times.
Instead they can show their key as QR on one device, scan it on second device and be ready to go in a split second - and it all works seamlessly, don't need second device online.
Yes, less secure, more chance of key leaking/stolen, but the UX is better.
Though have a desktop app that acts as both client (viewer), relay (exposed through a relay proxy and acts as backup) and signer, that can just run in the tray, is a good idea.