full tonneau? If it's a full tonneau then you can use it to cover the interior of your car while you're not in it with the top down. I've seen some that'll let you uncover the drivers side so you can drive and the passenger side will stay on. So its easier to cover the entire cockpit when you get to your destination. I would consider it a factory optional cockpit cover that you can't get with any of the modern cars. For example, if you want to cover the cockpit of a Miata without putting the roof up you'll have to get a cockpit cover to do it. With our Bs you have the full tonneau cover...
I wouldn't use it to store the car outdoors in thunder stormr, but its a handy thing to have for when your out and about, going to leave the car unattend but don't want to fuss with the convertible top.
I don't currently have one, but if I did I'd use it all summer long out here.