I really want to know more about the usecase of having to store SSH key on your watch.
Nah, me either. Especially as I'm in England. Please don't send the black helicopters.
Having something critical on your watch (PGP/SSH) means you're never stuck without it. I'd love to be able to plug it in as a bootable USB with Tails on. I seem to recall someone doing that for phones so it might be possible!
The reason I always need keys... Mum uses Linux (my choice) and I admin it from 200 miles away when she can't get her game to run.