It's certainly less flexible than a general-purpose Charm to convince people that you're telling the truth. But it has a number of miscellaneous advantages. For example: suppose the target later discovers the truth. What happens?
If you lied, and used a Charm to convince them you were being honest, then they now know you're a supernaturally skilled liar.
If you told the truth, and used a Charm to convince them you were lying, they now know that you were telling them the truth all along. Not only did you successfully deceive them, but it's apparently their fault, because they wouldn't believe you.
Suppose you're Cassandra, and you *want* to keep Troy from defending itself. So you go around telling people Greece is going to invade. They don't believe you. What happens when they find out they were wrong? Well, in a traditional Greek tragedy, they probably kill you anyway, but in a less structured environment, they may say: "Wow. We should have been listening to you all along, Cassandra. What should we do next?"
What it Does:
Jon looked flatly at me. "You're Anathema. No better than those monsters we just killed."
He'd seen my anima. He knew it was true. "That's right."
He heard it all in my tone: just how hard it would be, if he decided to kill me. Just how few people would believe him. The cost to his heart of killing a friend, for all the coming years.
He made the only choice he could bear.
"… damn it, Minna, *no*." he said. "What kind of trouble are you *in?*"
Maybe he had other reasons for hiding from the truth. I'd probably never know.