Previous poster is bang on. They allow any water inside the wall to drain out from behind the brick so that the brickwork stays dry.
ConcreteMasonary: I have brick veneered concrete block walls on my building. There are no weep holes at the bottom of the brick wythe at all, and I get efflorescence on my exterior brickwork every spring, especially near the top of the building where warm moist air accumulates. I believe that the efflorescence is from the melting frost that forms inside the wall over the winter.
What do you think of the idea of adding weep holes at the bottom of the wall, and the same 30 feet up at the top of the wall, and using the unavoidable heat loss from the building in winter to drive a convective air current between the brick and block wythes to evaporate any frost that forms in that cavity over the winter. Kinda like roof ventilation, only "wall ventilation".
It's not an intentional curtain wall. It's an "American bond" brick wall where every 2nd block is a 3/4 block and every 6th brick row is a row of headers to tie the two wythes together.
Would there be enough of a gap between the two wythes for this scheme to work reasonably well?