Why a secondary used to always be "required"
I haven't been brewing long enough to know when or where it started but it the general premise was that you had to do primary fermentation and once that ended you needed to move your beer to another vessel because keeping it in contact with the yeast for a couple weeks would make your beer taste like beef broth from autolysis (after yeast die they release compounds that have a beef broth-like flavor). The major reason given for this belief was that once upon a time homebrewers didn't get access to very good yeast so they died quite quickly. I suspect this is myth itself. People were brewing for thousands of years before the 90s/early 2000s and I've yet to see any ancient text that talk about beef broth-flavored beer.
It's more likely that homebrewers did what they still do: looked at what commercial brewers do and try to emulate it. Commercial brewers do rack to bright tanks after fermentation ends. However, commercial brewers operate on a very different scale. The amount of beer and yeast they have in a fermentor is substantially more than we carry in our buckets and carboys. The pressure from all that weight is different. Using massive cylindro-conical fermenters means all that pressure bears down on a relatively small cone of yeast. The pressure and weight increases the heat in the cone and that accelerates yeast death. There are several other reasons they use secondary vessels that we do not.
Some time in the mid-2000s some homebrewers tried leaving their beer in primary for several weeks and discovered no ill effects. Thus the no-secondary movement began.
Why a secondary is not required
Once we agreed that three weeks sitting on the trub would not create a big bucket of vegemite (a British food product actually made from autolyzed yeast that tastes a lot like beef broth), the tide started to swing towards an equally monolithic position that secondary is never, ever required (and its corollary myth that you need to leave your beer in primary for a month). However, there are some good points on why a secondary is usually not required.
Other than the dreaded fear that autolysis occurs almost immediately, a secondary was championed as the best way to get clear beer because somehow not having trub at the bottom of fermentor meant it could get clearer than in primary. That's sort of like saying a lake or river can never be crystal clear because there's rocks, sand and dirt at the bottom. You know that is not true. Similarly, you can clear out your beer in the primary just the same. Once heavy particles like yeast and proteins descend to the bottom of the fermentor they do not have the power to defy gravity nor is there a limit on how much heavy material can stay at the bottom of a fermenter. If that was true, liquids like whole milk would never naturally stratify. Same goes for those crystal clear lakes with miles of dirt and rock beneath it.
Another reason given sometimes was that you needed to transfer to secondary to add post-fermentation additions like dry hopping. Again, there is no basis for this premise other than a lot of people said it. Hops, fruit, oak, etc. work the same way whether there is an inch or a millimeter of trub beneath the beer.
You can produce delicious, clear, healthy beer with only your primary vessel.
Why a secondary is sometimes a good thing
There are a few reasons why you might decide to secondary your beer anyway:
- You want to yeast wash and add post-fermentation additions but don't want to fight through the additions to get to the yeast. It is a lot easier to yeast wash without fruit hanging on top of it. You also have the benefit of not having to worry about whether your post-fermentation additions might bring an infection. Although proper sanitation usually avoids that problem, it's not really easy to sanitize dry hops and I've seen a few infections related to them.
- You are making post-fermentation additions and your primary vessel won't fit the trub, beer and additions. This is pretty easy to do when you're adding fruit, especially if you are using a primary vessel with very little headspace to begin with.
- You plan on using a particular kind of secondary vessel, like a barrel. You don't usually want to do a primary fermentation in a barrel because you either chose to lose beer to blow off or have more headspace than suggested. Using it as a secondary vessel allows you to fill it full without headspace or blowoff problems.
- It is a strategic part of a sour or funky beer. Some sour beers are aged on the trub, like lambic, but others are racked to secondary, like Flanders reds. The presence of the trub can increase the funk and the absence can help emphasize a cleaner sourness. A secondary vessel helps limit access to trub.
- Similarly, it is a strategic decision to split beers and age differently. If you want a beer partially on one fruit and partially on another, again you'll need secondary vessels.
- You lager or cold crash your beer but then move it to the area you bottle/keg and in the process of moving the fermentor you are swirling trub back into suspension. The whole point of lagering and cold crashing is to get all that crap at the bottom of the vessel. If you are carrying the fermentor around and putting all that stuff back in suspension you're just wasting time and electricity. If that's a problem then consider using a secondary vessel to limit how much stuff can come back into suspension when you move the fermentor.
- You think your beer tastes better for it.