It's kind of used as a joke more than not, sort of a forum meme, let's say the topic is best music video ever, It would be humorous if I put Never Gonna Give You then /thread. It's saying this is the defining symbol of such and such, accept it and shut up. But when It Is used In conjuction with a legitimate argument, like If I said Land Of Confusion, you're right, it is arrogant, while my point would have some validity, saying /thread instead of, the social commentary is deep, or the visuals are amazing, I am being a bit of an ass.
Sorry to rant, It's just that In most cases, from what I've seen, its a joke.