The 5 gallon glass carboy has long been used for secondary fermentation when making beer and wine. As other sizes became available the home brewer adapted them for other uses. The 6.5 gallon glass carboy is often used as a primary fermenter offering the home brewer a view of the fermentation going on inside the carboy. The three gallon carboy has found a use for doing smaller batches or split batches of beer. However you choose to use the different sizes there is always a place in your home brewery for the glass carboy.