Sarcasm appears to be a group membership test mechanism. It involves saying something that is obviously untrue according to the speaker's group's beliefs as if it's true, with the expectation that members of that group will understand that the speaker can't possibly believe that, and nonmembers will show their non-membership by acting as if the speaker does.

Sometimes it's the other way round. The speaker says something with easily-recognised markers of not saying what they actually believe (e.g. call something a "modest proposal"), to cloak the fact that they are saying exactly what they actually believe. Insiders know what is being communicated, while plausible deniability is maintained to outsiders.