A paper passport can be valid for 10 years (maybe more, I'm not sure). It can be stashed in a safe. It can be left alone for several years and be picked up just

A paper passport can be valid for 10 years (maybe more, I'm not sure). It can be stashed in a safe. It can be left alone for several years and be picked up just before leaving for the airport. A smartphone will not satisfy any of these properties.