It's used for if you have a gamepad dock you can charge it on the dock via the port

It went unused, it was supposed to be for a Wii zapper 2 though. The 2 metal things next to it is how the gamepad charges when on the stand

It's used for a charging dock

The port was formerly known as a WUP-PR1 (Prototype) port used to connect the GamePad to the test Wii U systems, it could transmit basic sound and video data output. Afterwards Nintendo had a concept released for a Wii U zapper. The port could also be used to modify coding.