Sarcasm is the last refuge for someone who doesn't have a good answer...

It can be that, it can be for other reasons. As often as not that I see .. and certainly that I use it .. its a response when the other party is being blatantly untruthful, ignoring facts/data or being irrational. It can be a way of pushing back on the absurdity of that line of discussion, calling it out as disingenuous or dishonest.