What's the purpose of cooling the sensor?

CCDs create charge from incoming light (the photoelectric effect, for which Einstein got a Nobel Prize) but also from thermal energy. For very short exposures with plenty of light (the kids at a soccer game), you don't notice the thermal part of the image. But for low light applications like astronomy and fluorescence, you want to minimize thermally generated charge so you get a better image of light-generated charge. Cooled sensors must be housed in a sealed, dry environment in order to prevent condensation of moisture from the air.