Samsung’s latest DeX dock allows the Galaxy S9 to be used as a trackpad, a feature that Huawei already supported in its own PC mode and Microsoft in its Continuum before that.
Android P may bring official support for this and even add the option of using the phone as a keyboard. A patch for this was committed back in December 2016, but has been disabled.
Using a phone as a trackpad is pretty okay, though the typing experience won’t be great. Still, simplifying the controls to something like joystick sounds nice – e.g. an extra controller for your console. Alternatively, it could be a standard way to control “smart” appliances over Bluetooth – press any key to do the laundry.