Learning to homebrew
But, do you really have to have an airlock?
Most homebrewers take it as a given that they should have an airlock on their primary fermenter.
While it is certainly possible to brew beer without one, an airlock offers cheap insurance against infection, oxidation, and other potential problems.
Second, it helps keep a layer of carbon dioxide inside the headspace of the fermenter.
This layer prevents oxygen from entering the beer after fermentation - reducing the chances of off-flavors from oxidation.