Apple has released tvOS 11.4 for Apple TV and watchOS 4.3.1 for Apple Watch users less than a week before WWDC where it is more than likely tvOS 12 and watchOS 5 will be announced.
The key feature, when paired with iOS 11.4, is AirPlay 2 functionality. AirPlay 2 is designed to bring multi-room audio to the Apple ecosystem, allowing the same music to be played on multiple devices throughout the home. tvOS 11.4 turns the Apple TV into an AirPlay 2-compatible device that's listed in the Home app for HomeKit devices. AirPlay 2 playback can be controlled through the Apple TV, through an iOS device, or using Apple's personal assistant, Siri, to play content using voice commands.