Like Robert, I use the full tonneau all the time.
As he says, it protects the interior from dirt, sun, even light rain, and the curious.
It will keep you warmer on a chilly day if driving alone with the passenger side zipped.
It's also great for travel. Leave your bags in the car while stopped for lunch, no worries. And if you need to stuff in an extra jacket, the tonneau stretches more than the frunk would.
In a pinch, you could probably wear it as a rain poncho, but I've never tried that.
Maybe the most useful swatch of canvas in Christendom.