Rene Richie, iMore:
The iPhone has Touch ID, which means that it can require my fingerprint before authenticating anything from unlocking the phone to buying from iTunes to paying with Apple Pay. The Apple Watch has the ability to take Apple Pay authentication from the iPhone and maintain it for as long as skin contact is maintained.
That means both the iPhone and the Apple Watch don't just know that we have them, but who has them — no fingerprint, no skin contact, no authentication. And that makes them very, very interesting.
As Rene goes on to say, the possibilities are endless. From unlocking your Mac to unlocking your house, Apple is laying the foundations for improving both security and convenience.