Carboy caps work to cover carboys as an alternative for regular stoppers. They have two spouts, one to facilitate a blowoff hose and the other for an airlock.
However, they give you the ability to attach a racking cane directly through the 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 also agnostic in their applications for 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 carboys. The dark red carboy caps are for 6.5 gallon carboys.