It's the peak of summer and positively roasting outside. Worse, the parking lot doesn't have a single speck of shade to spare. In an effort to avoid barbequing yourself when you get back in, you position your sunshield so it helps deflect the hot rays. After running your errands you hop back in the car, pleasantly pleased with the sunshield's performance.
Somewhere, sometime, somebody must have been so happy about climbing into a relatively cool car that taking the sunshield off was just not a priority. Otherwise, there's no need for this legal warning: "Do not drive with sunshield in place."