For apps that integrate with the SDK (currently Youtube & Netflix, but I am sure more will be coming soon), the processing of the content can be transferred to the the Chromekey. Hence, once you "flick" your Netflix content to Chromekey, your phone is free and not processing content anymore unlike Airplay, which is a huge win for your phone's battery.
If that's the case, I wonder why Chromebooks (other than the Pixel) aren't compatible.