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The fan is to help cool the mirror down.. it's passive cooling, by blowing ambient air onto the mirror.
If the mirror is much warmer than the air outside, you get tube currents in the tube and a boundary layer of air above the mirror which distort and refract the light as it comes down your tube.
It is usually interpreted as bad seeing.
You may not notice it in an hour or two of observing because it can take many hours to cool the mirror down properly. During winter months it may never catch up.
The daytime/nighttime temps in Brisbane are pretty close too, compared to places like Canberra so the effect may not be as dramatic in your part of the world.
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