OK, speaking as someone who has recently added "NSFW" to some question titles, here was my reasoning:
- Some workplaces have (crazy, stupid) policies where even browsing to a page that has swear words on can get you a visit from the IT department (or worse).
- Some people really, really, don't like seeing offensive language. (Personally I don't have a problem with it, especially in written form - after all, it's just a word! - but if there's something simple and easy we can do to avoid offending such people, without detracting from the value of the site, so much the better.)
- For the questions I'm talking about, someone who wasn't aware of the British usage of those words - or even someone who was - would not necessarily expect the questions to contain the very strong language that they did. (It's perfectly possible to ask the question - and answer it - without using stronger language. That didn't happen, and there's nothing wrong with that, but it might catch someone out if they weren't expecting it.)
Therefore it seemed reasonable to flag the questions in a way that would let people working for organisations with over-strict policies, and people who dislike such strong language, know what they might be in for.
(There doesn't seem to be a consistent tagging scheme at the moment - I've seen vulgar, offensive, insults, swearing. I agree tags could be used in this way, but as has been pointed out, the tags don't necessarily appear in all the places where the title might...)
nsfw
should not appear in question titles. Anyone serious object if I start trimming them out?