What does the temp gauge do anyways?

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As some may or may not know, my temp gauge doesnt work after i got my clutch replaced... someone told me some very detailed information on how to fix this but i havent found the time to do so between school, work, and trying to stay warm in this damn Indiana weather. Anyways, what exactly does it do besides just tell you if your car overheats or not? Does its function have anything to do with how the car runs? When its cold, it will idle high to warm up, and after warm it will go to