I think the Apple Watch will be marketed as "a secure key to automatically unlock everything in life" (physical and digital). Think about this:

A smart watch's security is arguably stronger than a smart phone and is completely automatic. You type in a code once after you put it on and it stays unlocked all day long - ready for immediate use. If you remove it from your wrist the sensors will detect that and it will require that you re-authenticate. The proximity between an unlocked watch and it's owner is 100% accurate - where a smartphone can be left on a desk, in a purse, be stolen, or otherwise be away from it's owner - locked or unlocked. The watch is automatic, consistent, and secure without having to be re-authenticated for each use. Because of it's proximity to the owner and convenient security it will unlock many new use cases. It could automatically unlock everything from credit card payments, debit card payments, car doors, home doors, gym or event memberships, temporary hotel room access, proximity detection lights, computer or online profiles, online payments for nearby devices, access apps like 1password, and much more.

Some of these use cases could be done with a smartphone, but with a smart watch it's 100% automatic and arguably more secure. You don't have to pull out a phone, publicly punch in a code for each use, or use Touch ID (a relatively weak security method that was built more for convenience than strong security). You could just walk up to a computer and it's fully unlocked while still being 100% secure for the owner.

The impact of this could be massive in the not too distant future. I could see this replacing passwords, keys, and virtually any other use-case where some kind of identification or authentication is needed. No more typing passwords online, no more having bank pin numbers, and no more physical keys. It could all work with auto-generated tokens and far more security.

I'm guessing this will still take 2-3 years to go mainstream in any meaningful way (iPod, iPad, and iPhone all took 2-3 years to really go mainstream), but I think Apple is positioned well to enable a lot of this functionality right from the start.