The other week I was driving to pick up my kids from Grundschule and I spied not one but TWO offenders of this outdated practice. My first experience with the tradition of hanging your comforter out the window was when I landed in Germany, about 8 years ago, and was staying at the Hotel Garni in Rodenbach. When the housekeepers cleaned our room, they stuck our comforters out the window. Due to my ignorance on German culture, I thought that these comforters were special somehow and required to be flung out our dirty window. Of course I didn't want to look foolish, so I never asked why they were doing it, just followed suit when we moved into our own house. I felt so "German".
Initially I assumed it was probably just a practical place to put the comforter while the room was being cleaned, as Germans tend to be very practical. As many things are in Germany, this act is actually rooted in tradition with a base of practicality. It has been traditionally accepted that the comforter, or decke, must be aired out. When the most popular sort of decke was the daunendecken (made of down), there was a need to regularly air them in order to prevent mold. This housekeeping necessity of yesteryear is now unnecessary, as the materials in the decke are more than just down. This knowledge doesn't stop tradition. So, you can air out your comforter if you'd like, but its simply not necessary. In reality, your bedding is probably just getting gross stuff on it from the side of the house. However, if you have a comforter made of pure down, carry on!