Are they acid etched into the glass? It's kind of like a theft recovery system. Those numbers are unique to your car, and if it were stolen, even if all the VIN plates were taken off the car, they could still ID it as your car...or parts of your car through all the glass.

The reasoning behind it is that glass can be expensive to replace in the whole car, so people might not be as likely to destroy all of it if the car is taken and chopped up.