I used to use it on my other account because someone used to PM me every time I was online asking if I wanted to trade them even though I said I didn't. Very obnoxious.
No. I don't think I've ever used it. I guess it's so people can know I can trade, if I have any pending agreements. I don't mind if people are invisible, though. And I'm really glad no one here has been annoying enough to prompt me to use it.