Like James, I have also wondered. One hypothesis was maybe it marked standalone novels from a series, but I think that didn't hold up.

It's a market signal -- it tells people that the book is upmarket or elevated.

You know, I think it actually works like that on me. For some reason, I kind of like it, and you might have identified the reason.

I guess it would be interesting to get a nice big list of examples. I could see something like Byatt's Possession being marketed this way. I'm not so sure about Butler's The Fledgling, but I guess it could be at work there, too.