99 Chev Tahoe rear vents

Does anyone know what the rear vents on the upper part in the rear are for, looks or what? If they vent to the inside where do they open too?

The ones above the back seat are for the rear air on my 99 Yukon. There are a pair in the middle of the roof and one on each side right above the doors. When you turn on the rear air, they blow cold air. If you don't have rear air, I guess they're just for show!