It's called changing the argument. If you don't have something good to say then you derail, derail and derail. You get them talking about other things. At the very least, it confuses the argument. It's especially effective if you say something outrageous or stupid that causes the other person to feel it's so idiotic or crazy that they have to respond.
The question is why people feel the need to do so on a message board that is a casual conversation. You don't really have winners and losers like a debate or a political election.