That's your thermostat. You could technically run the vehicle without it, however I recommend getting a new one if the current one is faulty. Running without a thermostat means that your engine will not get upto operating temperature as quickly.
Before that, you need to find out why the radiator fan is not automatically turning on. Does it work when you you turn on your A/C? Also, check the cooling system to see whether there are any blocks/leaks.