When should I delete an answer? has an early exploration of a rationale for when answers should be deleted by 20k voting. Unfortunately, time marches on and the reasons explored in the sole answer basically map perfectly to NAA or VLQ flagging reasons, or at least Recommend Deletion in LQP. (They're also mentioned in the Help Center anyway now.)
But I'm pretty sure 20k answer deletion is supposed to be a little more than just "you can now directly vote to delete what you used to have to review-vote, and sometimes that means it gets deleted faster". What are some examples?
- Late bad semi-dupe answers? (These are hard to reliably push through with VLQ-flagging, but not impossible, and ♦ flagging usually works.)
- Trolling/non-serious answers? (For example, a ridiculous definition of a word on ELL that might embarrass someone if they took it seriously.) In some ways, these might be considered similar to the various PHP security anti-pattern answers we've dealt with on SO, since most of those with any claim to expertise in the subject would recognize them as bogus, but anyone credulous enough to rely on them will find them actively harmful (if they ever find out at all). The difference is that the wrongness is generally a lot more obvious to ordinary semi-experts, not just the rockstar security consultants, so there's no question of deleting/modifying a highly-scored answer. Of course… that arguably makes them nearly toothless. Hmm.
- Answer that blatantly ignores one or more constraints in the question, like a Java answer on a C# question, a D&D answer on an Exalted question, or the like? (There are usually arguments made that these don't fit NAA by the letter of the law, and VLQ is not terribly obvious either, but LQP reviewers usually do the right thing anyway.) Much like joke answers, these usually don't require more than a small amount of expertise in the subject.
I can't think of anything else, and I'm not convinced the first one is actually worthwhile. So what I suspect is that while there are a number of situations that delvoting works well in, they are sufficiently unusual it's hard to find centralized references to them. I'd like to know which ones those are, so I can run more effective routine searches (on the site I have the priv, I check 10k tools once a day, normally) or just keep an eye out.