part of the gamepad is a controller . another part of it is a screen , and another part of it is tied to the wiiu , it isnt just a controller it carries a feature set integral to the wiiu's functionality.. part of that being a gamepad.. get it.?
Or you can simply look at it as a sophisticated controller, the screen being a part of the controller, and the fact that it's tied to your Wii U means little. The Wii U is capable of having two game pads linked to it, and you can turn on and navigate the Wii U without the game pad. Whoever said it was a necessity in the same way as a steering wheel is necessary for a car to drive was using a poor and incorrect analogy (though yes, there are a handfull of games you can't play without the controller, the vast majority of games have it strictly as optional).
A more apt analogy would be that the game pad is like cruise control in your car. A feature that comes built into your car, but you don't ever have to utilize it if you don't want to despite the benefits it does give you when driving.