Why do the Germans switch their tires twice a year?

Here it is again, time to switch car tires to the winter ones. All over Germany people own two sets of tires, one for summer and one set for winter. And they all switch them usually around the same time. I am not certain why they do this. Do they not know of all-purpose tires, tires that work in all seasons?

I, frankly do not see much difference between the winter and the summer tires. The thread depth and width look the same and they are identical in size and shape. Having two sets of tires and switching back and forth does keep them lasting longer. Perhaps, thriftiness was the rational at one time, or tire manufacturers proposed it so that car owners always had to buy two sets of tires instead of one.