Apple Watch and your Mac (2013 and newer) can work together to keep everything on your computer a little more secure thanks to watchOS's Auto Unlock: It lets you automatically unlock your Mac (no passcode needed) if you have your watch on and unlocked.
But if you have an Apple Watch Series 3, you'll need to upgrade your computer to macOS High Sierra for this feature to work properly — at least for the time being.
Why? The Series 3 Apple Watch uses the company's newly designed W2 wireless chip; it makes Wi-Fi and Bluetooth faster and more efficient, but requires updated software on the Mac so that the two devices can talk to each other.
Currently, the W2 chip is only supported in macOS High Sierra, which means that any Mac you might want to use with your Apple Watch is going to need the update to function correctly.
But what if you don't want to upgrade to High Sierra? Unfortunately, we don't currently have a roadmap for when this software tweak might arrive on older versions of the software (in the form of a security update, most likely). As far as we can tell, there's no technical reason why Sierra wouldn't be able to receive a patch to let it interface with Series 3 — both macOS 10.12 and 10.13 support the same computer models. More likely: This update fell down the priority list in the wake of other, more potentially destructive macOS bugs.
If you need to continue using Sierra for app-related reasons — or you're a little wary of upgrading given the aforementioned destructive macOS bugs — you can still use Auto Unlock if you feel like partitioning your drive: That way, you can keep some of your work in a Sierra-only space, but get the everyday benefits of using High Sierra on the other partition.
Otherwise, it's about playing the waiting game. There are still plenty of great things you can do with your Series 3 Apple Watch — but if you have macOS Sierra, unlocking won't be one of them just yet.
Updated March 2018: Added the latest information about Series 3 and macOS Sierra.