It's for the Wii U Gamepad Dock. It's just another way to charge it.

Gamepad Charging dock

its actually an accessory port! it was intended to be used as a connector for various accessories, but since the wii u did so poorly in sales, it went entirely unused; there wasnt a single game or attachment created that utilized it.

this isnt a totally uncommon practice for nintendo, as several previous consoles had extra ports that went entirely unused, such as multiple extra serial ports on the Game Cube, and a port on the N64 for the scrapped disc drive!