Ryan Christoffel at MacStories:

One particularly interesting detail about Express Mode is that it enables a special Power Reserve state for your iPhone or Apple Watch. When your device is set up for Express Mode, and its battery is close to depletion, iOS and watchOS will automatically save a certain amount of power so you can still use your device for transit access for another five hours after Power Reserve kicks in.

Even if my phone is almost fully charged, I get anxious about its battery when relying on it to get back home, to my car, or to a hotel. I like the idea of a power reserve for showing transit system tickets.