You have to get some fresh air.

In part it's a bit of housekeeping, where - especially during winter - you'll have a fifteen minute period at the height of the day where they'll throw open all the windows and shutters to get some fresh air through a house. It's actually really good for ventilation, since German houses are very, very good at keeping air in, which includes all the huff and puff and condensation that leads to mould in corners and behind sofas and such.

The other part is that the Germans are, by and large, very good at harmless tradition and keeping little aphorisms alive without question. The cultural insistence on fresh air as a curative and solution to any ailment, problem, social ill or minor irritation goes back hundreds of years and I can tell you from direct experience that nobody knows why they're so adamant about it, but by god, you need some fresh air, how long has that window been closed, c'mon let's get that open and have fifteen minutes of frische luft schnäppchen or the ancient, quiet ghosts under the roots of the hazelnut and chestnut will open their eyes to come and find you, and you don't want that.