It is required when leaving the Schengen area. They like to know when people are leaving so that they know who is overstaying and who isn't. The airlines want

It is required when leaving the Schengen area. They like to know when people are leaving so that they know who is overstaying and who isn't. The airlines want to see the passport too, but every time I've left the Schengen area (Germany, Netherlands, and Iceland), the national authority for that country checks my passport and stamps it with an exit stamp showing the date, time, and airport of departure. That doesn't happen when leaving the United States: just the airlines want to see the passport to make sure you won't be immediately rejected at the destination.