In fact, the border case is just an edge case. You also want your accounts protected if you lose your unlocked phone, have it stolen, have your password stolen,

In fact, the border case is just an edge case. You also want your accounts protected if you lose your unlocked phone, have it stolen, have your password stolen, log in on a dodgy public computer, plug in an infected USB stick, or leave your laptop unattended in the wrong hotel room.