It has to do with keeping your poise as much as it does having the best “facts.”
The important thing, I think, is to remember that winning a debate is not the same thing as persuading someone.
Debates are about getting in a good quote or two – and pushing the right buttons.
The more outrage there is, the more you will know that all good logic and reason has left the dialogue.
When people realize their arguments are not rational, they attack the messenger on the other side. If you have been well behaved in a debate, and you trigger an oversized personal attack, it means you won. When people have facts and reasons in their armory, they use them first. When they run out of rational arguments, they attack the messenger. That is the equivalent of throwing the gun at the monster after you run out of bullets.