The Apple Watch can be used to make payments with Apple Pay. How it authenticates the user is that it “registers” the user through the heart rate sensor underneath the watch, and as long as the user has entered their passcode and the watch remains on their wrist, they are considered to be authenticated.
It can also be used to help Mac users log into their computers, giving them an alternative to logging in besides the use of passcodes or Touch ID (found on newer MacBooks). Now according to 9to5Mac’s Guilherme Rambo, Apple might be looking to expand Apple Watch authentication with macOS 10.15.
Based on sources familiar with the development of the next major update to macOS, it seems that Apple might want to allow the Apple Watch to authenticate users beyond just logging into their machines. It is unclear as to what this could entail, but it has been suggested that whatever Touch ID can authenticate at the moment could be replicated with the Apple Watch.
Presumably, this means that it could let users use Apple Pay on the web, log into websites, and so on. Previously it was also rumored that macOS 10.15 could see Apple introduce a new feature that will turn the iPad into an external display. Take it with a grain of salt for now, but we should have more details come WWDC 2019.