Well this is an old thread so I'll take a stab at it.
You kind of answered it yourself, its to map additional functions that you need, but on console there aren't usually extra functions to worry about, PC is different as there are way more functions to map then buttons. So its for the extra buttons and the easy access to them.
Button boxes are typically only useful on PC, there are typically tons of functions to map in PC games.
For GT all I can think of is to program it for the real time adjustment (RA) function which eats up a lot of buttons.
The other factor here is that button boxes dont work with consoles, this one works on console because its made using a Playstation controller.
A few years ago my PS3 "button box" was just a controller mounted in a handy spot, and all I used it for was the RA menu. I really didn't end up using it much so I took it off.
