Some that I, personally, have used include things like "they're pretty crude illustrations though since I am not really an artist" (in reference to an illustrated story, because false advertising in re: artistic quality is nobody's friend!), or "once you create the rest of the world it has to count as much as your special country Mr. Lewis" (in reference to a Chronicles of Narnia story set entirely in Calormen from a Calormene viewpoint), or "minor background creepiness on account of Gamzee" (because there isn't a tag for that, but when the point of the story is fluffy cuddles and snuggles, it's nice to point out a potential mood killer), or "there is not actually any sex in this story" (to qualify a canonical "Threesome" tag, because again, false advertising is nobody's friend).

In other words they're little bits of additional information to convey details about the story -- mood, background assumptions, attitude toward canon, etc. -- that help people decide whether any given work matches what they want to read.