Bottle fillers allow you to fill a bottle from your kegerator or keezer setup. The advantage of these is that they equalize the pressure between the bottle and the keg that contains the beer. By equalizing the pressure, you eliminate or greatly reduce the foam in the bottle and are able to cap the bottle fully carbonated. You are also purging the bottle of air so you don't have to worry about oxidizing the beer. This is a great way to prepare beers for a contest or to take a few beers to a friend's house.