Big mouth carboys came out a while back, intended to help homebrewers who dry hopped. With the normal, narrow-necked carboys, dry hopping was very difficult. I've never dry hopped in bags, so I always poured my pellets down the neck of my carboy. The idea of trying to fill a hop bag and stuff it down a carboy neck always struck me as ridiculous. Enter the big mouth bubbler.
The mouth is nice and big, allowing you to get your entire hand and arm inside the carboy. I still went ahead and hopped with loose pellets rather than using a hop bag, but if I ever dry hop again, I think I'll use a bag. Much less likelihood of getting hop residue in the finished beer. The price of these carboys is about the same as normal carboys, and I've read that Korean supermarkets in California sell them for about half of what you'd pay in a homebrew store. I wouldn't be surprised if these were originally intended to be for making kimchi, and some enterprising homebrewer saw a use for them in brewing.
- Makes dry hopping much easier.
- Wide mouth allows arms to get inside carboy, so that it can be used for other things like winemaking, or fermenting kimchi and sauerkraut.
- Much easier to clean than normal carboys. Can wash by hand, no need to use bristle brushes.
- Requires a harness to carry it when full.
- Requires periodic replacement of the lid gaskets.
- Lid tightens very securely, which can make it difficult to unscrew, especially with wet hands.
- Slightly wider than normal carboys, takes up more floor space.
- Requires a smaller stopper than the universal stopper used for normal carboys - if you don't have that size on hand, you'll have to buy new stoppers.