Yes its to get rid of the musty smell when it occurs. You generally don't need one until the car is at least a year old. You dont need them ever if you are disciplined enough to use the aircon on hot occasionally to dry it out and kill the bacteria. But once you get "the smell", this is pretty much the only fix.
They are only on sale From Monday.
You use it by starting the car, putting the aircon on full, on recycle air, and open all the internal vents. Then (having covered any leather) you set the can spraying (it wont stop until its empty) and set it in one of the drinks holders, and shut the door.
The contents of the can circulates around the cabin and all the aircon./heater pipes. After some time, around 20 mins (the can can spray for 5-10 mins) You get in, switch off, reset the heater settings, recycle the can and get sweet smelling car again.
Definitely don't do it while the car smells new!
