Valve released the, and while some people chose to use the controller to play their Steam games on PC (others may prefer their keyboard), it apparently had one dormant feature all this time: Bluetooth support.
Years later, Valve is now enabling Bluetooth LE support in its controllers, which means it had the hardware for it all along. (Normally, you pair the controller to a wireless USB dongle.)
This comes right before the May 21 release of the PC games on your phone (or smart TV) over Wi-Fi. The company says that Bluetooth LE is required for using the Steam Link app with your controller., which will let you play your library of Steam
Controller support might also make it much more natural to play Steam games on your phone.
For info on how to set up Bluetooth controller support, you can check out Steam's FAQ.