This is a mechanism to create a persistent connection between a second screen and an HbbTV device using a cloud-based service. The TV-based teletext overlay has a button which can be pressed to show a QR code. the ids for the devices are paired on a remote web service the teletext app...
In regards to why you would use Apple Watch with iPad, there are reasons. For me personally, I'd like to use the watch as a remote control for the iPad. But I know that's unlikely what it will do. So, I'm just saying there are ways you could use the watch with the iPad, ...