The point of a secondary fermentation for beer is to allow it to condition after the primary fermentation is complete.
Moving the beer into a secondary vessel prevents the yeast inside the beer from producing certain off-flavors and allows the brewer to clarify, dry hop, add flavoring, or age the beer more easily.
Secondary fermentation is very helpful anytime your beer will benefit from extended conditioning time whether it be to allow it to clear, to add extra flavor, or to let flavors mellow and blend.
A better way to think of secondary fermentation is to simply call it the conditioning phase. This captures any of the reasons that you would normally use a secondary fermenter in the first place.
- Reduces off-flavors - Clarification - Dry hopping - Flavor additions - Long-term aging and conditioning
First, there will be no active fermentation inside the secondary vessel so there is no need for extra headspace. Secondly, because there is no carbon dioxide produced in the secondary,
Since there is little or no active fermentation happening during ‘secondary fermentation’ you won’t typically need an airlock although it is perfectly fine to use one.
You can skip secondary fermentation for your beer if it does not need extra clarification, dry hopping, flavor additions, or extended aging.