What’s the purpose of cooling the sensor?

Lower noise and improved cosmetics. CCDs create charge from incoming light but also from thermal energy. For very short exposures with plenty of light (bright field microscopy), you don’t notice the thermal part of the image. But for low light applications like fluorescence, you want to minimize thermally generated charge to get a better signal-to-noise ratio in the light-generated charge. Cooling also minimizes dark current from hot pixels.