What is the function of the 'Retarder' (Dauerbremse) in heavy passenger motor coaches?

1. It cools the passenger cabin air conditioning system
2. It speeds up the bus on flat highways
3. It provides wear-free continuous hydrodynamic or electrodynamic deceleration, relieving the mechanical foundation service brakes and preventing brake fade on long Alpine descents
4. It plays recorded announcements through the speakers