Apple already allows MacBook owners to unlock the Mac using their Apple Watch. With the macOS 10.15 upgrade, Apple will further expand on this functionality and use the Apple Watch for authentication purposes in more scenarios.
On MacBooks with Touch ID, Apple uses the fingerprint sensor for authenticating Apple Pay transactions. Owners of Macs which lack Touch ID can also use their Apple Watch to authenticate Apple Pay transactions on the web by double-clicking the side button.
Apple is now seemingly planning on expanding this functionality further in macOS 10.15. It is unclear as to how or where Apple will further expand the use of Apple Watch for authentication purposes. It is possible that we will see Apple use Apple Watch for authentication purposes in all those scenarios where Touch ID is currently used on supported Macs.
Apart from macOS 10.15, Apple will also have to update watchOS on the Apple Watch to add this new feature. Apple will be revealing more details about macOS 10.15 at WWDC 2019 which is scheduled to take place in June.
By using the Apple Watch for authentication purposes for tasks done on the Mac, Apple is further improving the integration between devices in its ecosystem.
Personally, I would like to see Apple work on allowing Apple Watch users to lock their Mac right from their wrist. In my opinion, that would be a more useful feature to have. Or imagine using Siri on your Apple Watch to do something on your MacBook.