I think it's there to turn the car on and off

 

That'll be it!

 

haha.. I walked right into that... I am just use to my Rivian

 

It can also be used for rebooting/restarting the computer, just like on your desktop, laptop, tablet, etc.

 

Except that it doesn't really.

You can reboot the MMI by holding its power button.

You can reset some parts of the car by powering off, locking and leaving it for 30s or so.

There's also a "deep sleep" procedure that will shutdown and reset even more of the car.

Also, fun fact, shutting down a modern Windows PC doesn't actually fully shut it down and reset it (the kernel is hibernated).

 

The GT will automatically start when you hop in with the key and press the brake pedal.

I believe it will also turn off if you stop the car, hop out and lock it.

The button allows you to override that behavior - if you hopped in but don't need to drive so you want to turn the car off. Or you've arrived but want to wait in the car with it off.

It also allows you to turn off the car in an emergency:

If necessary in an emergency, the drive system can also be switched off while driving using the emergency off function*.
> Press the [START STOP] button twice in a row or press and hold it one time.

 

I figured it would be for the above^

You technically never have to even touch it in normal use, even to turn "off" the vehicle. Just put it in to "P" walk out of the car and lock it, the vehicle turns' itself off.