Its purpose was to expose the credulity and scientific illiteracy of many of the people who call themselves climate sceptics. While dismissive of the work of the great majority of climate scientists, they will believe almost anything if it lends support to their position. Their approach to climate science is the opposite of scepticism.
I think another purpose could be similar to a reason for challenging denialists to bets - it will reduce the number of ridiculous claims because the denialists will have to hesitate before backing whatever piece of nonsense comes their way. That's the idea, at least, but as with the non-betting denialists who won't put up and won't shut up either, it may not always work.
I also feel it's unfortunate that the somewhat deceptive technique of a spoof is necessary, but Rush and friends have just proven that's the case.