As I mentioned it is a breathing vent. These usually have a kind of mesh inside them. As the transmission heats up it allows the air to escape and traps any oil mist from making a mess over time. When the transmission cools and the air is suck back in it filters dust from entering over a period of time.
Sometime these things are replaced during a major service like 100,000 kms. I have seen these vents block up with crap mainly in dusty environments. Pressure can build up enough to cause some oil leakage from the seals, pretty rare though.