What is the Volkswagen solar panel 1CO 915 687 used for?

It is a low amperage, DC voltage (up to 18.8V) trickle charger. It is only meant to be connected when the ignition is in the Off position, and is to be disconnected before starting the car. It's meant to accommodate some of the power drain from peripherals, and it helps to perk up the battery. It is, however, not a great way to charge a dead or dying battery due to its low current rating.