But it's also bigger than that. If everything you do user-land can be synced down fairly instantly to any piece of hardware - phone, tablet, desktop, TV, watch, etc. - it provides a level of mobility and usability which can enable some very powerful use cases.
Approaching a device and have it immediately be "yours" is important for the screens in the self-driving ride you hail, or the shared workspace you might rent by the minute, or even the TV you sit down in front of in your own living room.
But this could even extend to the POS terminal which you checkout with at a store, a screen you walk up to in a mall, a digital assistant you approach in a store, an ATM, etc.
FaceID is transparent walk-up/pick-up authentication, which is table stakes for some very cool possibilities.