Titanic, the ship magnificent states the following about it:
“On each side of the recess was a cupboard fitted with glass racks and enclosed by a glazed door at the top and an oak-panelled door at the bottom. Above each cupboard was a brass grille over a ventilation opening.”
Pictures from the Olympic also showcases that curtains would block visibility when you try to look into it from the smoking room.
From that information I got I estimate that it served to let air trough since it was also connected to the ventilation shafts. Altrough I am not certain about it.
I hope however that my information helped.