I know what you're talking about, but have no idea what it is for. I know it isn't for defrosting the windshield.
Someone must know what it's for. I have ruled out HVAC vent, and I can't believe it is just a useless design.
If I were to hazard a guess, I would think it may have something to do with the airbag. Maybe it routes heat from the explosive charge away from the passenger