One: to raise the boiling point and lower the freezing point,
Two: to prevent corrosion and electrolysis

Your car's radiator needs to be clean to be cool. As time goes on, your inhibitor breaks down, your car's radiator builds up solid deposits that can clog the cooling system and damage can be done to your aluminium components, such as your water pump. A quick, inexpensive radiator flush can keep the system in shape.