A Faint Sound

I recently watched some sitcom reruns and noticed, for two shows in a row, that there was a situation where a character had to break some bad news and told the other character to "sit down." This was (and still is--also saw it later on The Big Bang Theory) a dramatic convention, but it's also occasionally used in real life.  But I must say, I don't get it.

The idea, I assume, is the news is so bad the person who hears it might faint so if she's sitting down she'll be safe.  But really, it's exceedingly rare for that to happen, or even for someone to feel weak at the knees.

In any case, hearing "you better sit down" is scary enough.  I think I'd rather have the bad news without any preparation. Worse, I've heard people break news that wasn't so bad (or that ends okay) with an implication I'm about to hear something awful.  Don't ever do that.  Unless you've got something so completely horrible you think it needs a preface, just get into it, and you'll probably be doing the listener a favor.