[Teutonic]

Just bought other two today for my other cars.
It is the holder for scented cartridges that makes you car smell nice.

Plug the rubber things on your vent (the vertical adjustment will be impaired on this particular vent), make sure you have a cartridge inside, switch it to 1 and turn the vent ON. You will love it.
Cartridges with different scent can be bought from BMW parts Department.

See my driver's side vent.