For a 2014 Expedition:
Its a mix of facts here. If the OP is right and the button on the liftgate "does nothing" then dewey is right ... button prolly bad. But Dorzak is right in that you can enable/disable rear power liftgate control from that button on the liftgate. If you "turn power liftgate OFF" in information display, you can still power open the liftgate from the button on the instrument panel or the key fob, and the button on the liftgate will activate the latch so you can open the liftgate manually.
I dont like all the power features (more unnecessary things that break & cost $$$) so I set my power liftgate OFF in information display. I can still activate the power liftgate from the key fob or the button on instrument panel (and it power opens/closes). When I press the button on the liftgate, the latch clicks and I can open the door by hand, the old fashioned way.
This is on page 113 of my 2014 Owners Manual.
(which is a piss poor description btw)