This is possible because of the "Project to this PC" feature available on Windows for devices equipped with Miracast ...
Starting with Windows 8, the Wireless Display Adapter (WDA) lets a Miracast-enabled Windows PC, tablet or phone mirror its screen on the connected screen via Wi-Fi. Starting with the Anniversary ...
Open Settings Select System settings Open Display settings Scroll down till you see Multiple Displays Locate Connect to a wireless display ... stream or duplicate my Windows computer on to Xbox ...
While originally designed as a remote access program, you can use Splashtop to turn your iPad into a secondary display for your Windows PC with a wired or wireless link. It's very similar to spacedesk ...