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I'm just a begginer too but I believe it is because if your mirror is warmer then the air around it, air currents will rise (warm air rises) from the mirror and causes turbulance which effects the light travelling through the scope.
So yes, the aim is to cool the mirror down to as close to ambient temperature as possible, not to just keep it 'steady'. Warming the mirror is probably the opposite of what you want to do. A fan not only cools the mirror down faster but it would also blow away the warm air that forms around the mirror before it has a chance to accumulate too much.
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