Engine "Coolant" which generally is Ethylene Glycol, is not an actual coolant, its the water mixed with the "Coolant" that does all the cooling.
Water however has disadvantages such as its high freezing and low boiling point, plus its good at causing corrosion. Which is where Ethylene Glycol or any other "coolant" would come into play. The "Coolant" lowers the Freezing point, raises the boiling point and has corrosion inhibitors.
You don't really need it in a PC's cooling system, but it is a good idea to use some sort of corrosion inhibitor.