Carboy caps work to seal carboys as an alternative to plastic or rubber stoppers. They have two spouts, one to facilitate a blowoff hose and the other for an airlock.
However, carboy caps give you the ability to attach a racking cane directly through the center of the carboy cap. Combined with a CO2 tank or other method to slightly pressurize the vessel, you can pump your homebrew easily for racking into another container. This makes it a dual purpose tool to add to your brewing setup.
Carboy caps are useful for making almost anything including beer, wine, mead, or soda. Be sure to grab the correct sized cap for your carboy. Orange carboy caps are for 3, 5, and 6 gallon glass or plastic carboys. The blue carboy caps are for 6.5 gallon glass carboys.