Just for fun, a few years ago I picked up an external speaker that I mounted on the wheel well under the hood. $8 at Radio Shack. It is hooked in to the PA connection of the CB. I don't really have any use for it, but my son has had fun with it when stopped at a light. He makes strange noises and gets funny looks from people trying to figure out where it is coming from, in addition to practicing to become a tour guide at Busch Gardens. My favorite use for it is at the drive through of the fast food places. I intentionally garble my order very loud and confuse the order taker. Oh, and if it is raining and you don't want to open your window, just use the PA!
I suppose some practical uses would be if you needed to call for help, call out for a lost dog or child. Want to tell that $%#$@ driver in front of you what you think. (Good chance you will get shot or arrested though.)
I believe all of the "real" reasons for having one have been mentioned.