It's not always possible to pop off with your mobile when you need to have a private conversation. Sometimes you're chained to your desk, pretending to be working when a delicate call comes in on your office phone. You could try whispering into the receiver, or using coded talk, or you could get one of these Open Area Telephone Privacy doohickeys from Hammer Schlammer, which creates a wall of sound around you and your phone (using your own voice, which you need to record in advance of use, natch) so those neighboring Nosey Nellies at the office won't be able to make out the nature or content of your conversation.
Personally, none of my secrets are worth the $399.95 price tag, but if I did have one, I think I'd use it all the time, whether my phone chats were confidential in nature or not. Air of importance and mystery and all that.
From: Sony Ericsson's first Android phone: meet the Xperia X10, out in 2010