The manufacturer has studied the catalyst to accept fumes of different types with heavier elements.  As a result if someone does not use AdBlue the engine needs  higher mechanical energy in order for the fumes to pass another type of hives of the catalyst.  This practically means:

1. higher fuel consumption 5-7%, thus higher financial expense.
2. quick damage of the catalyst because of blocked hives.
3. loss on the "best function" and performance of the engine
4. higher burden on the atmosphere.
5. expensive fines in case of fume measurement.