I found this here:
Air suspension info
But there wasn't really any conclusive answers.
That makeup hose could be for the dealer to refill the system if there is was a leak.
I'm still not convinced this mixes outside air with the Class 5 Nitrogen already in the system. It would not be a "closed" system if it did.
An air conditioning system refrigerant line is closed and "that tight" and never needs any Puron or R-134 added unless there is a leak.
FYI, this isn't to start some sort of forum war, but this is a good discussion on a system that we should all have a better understanding on since someday it's going to fail and need fixing.