Why does iCloud ask for my iPhone's passcode when signing in on OS X Due to some iCloud struggles, I had recently occasion to sign into and out of iCloud on OS X a few times recently.  Interestingly, when I have 2FA enabled on my iCloud account, OS X asked for the screen lock passcode to my iPhone when signing in to iCloud, and would not sign in without it (no .  After the initial sign in succeeds, it prompts for the iCloud password again, and this time asks for the 2FA code.  

This was a surprising and unexpected part of the authentication flow.  Does anyone know why iCloud / OS X / iOS interact in this way?
It just seems to be an extra security measure, so that somebody who steals your iphone in an unlocked state can not quickly use the 2FA to change your iCloud account.