I'm surprised so many of the comments jumped right to presenting ID to the government. This sort of thing was exactly what I had in mind too. My watch can already:

- stream music when I go for a run in the park

- give emergency info to first responders if something happens to me while I'm out

- pay for the bus or train home if I need it for some reason with Omny + Apple Pay

- pay for things I stop to buy with Apple Pay

Giving govt-issued proof of age so I can buy a beer on occasion after a workout without taking my drivers' license with me is a welcome addition to all the rest of that power.

If I'm crossing a border, I've already got alternative hard-copy ID on me.