Water molecules H-bond to each others. H-bonding is one of the strongest intermolecular types of bonds, so it takes a more heat to brake this type of bond (that would be the case when water boils). This also means that water has a very high heat capacity, which means it can "ëat up" a lot of heat before changing its properties. Adding antifreeze (ethylene glycol), or water wetter, or any other alcohol based solution, will decrease the heat capacity of water. Molecules of the alcohol will "brake the H-bonded water lattice and will lower the "strentgh" or capacity of the water molecule lattice. This will result in a decrease in heat capacity.
However, adding antifreeze will keep your reservoir water from freezing, if you go below 0 C in water temps. It will also help keeping the water clean with its toxic preperties. It will kill bacteria, fungi and the like breeding in the water, on the walls of the radiator/waterblock. You dont want these creatures thriving in your system because they will build up on the walls of the mentioned parts and lower heat transfer between the moving liquid and the hot/cold metal. Water wetter has the same use, its an antibacterial solution. IT DOES NOT MAKE WATER STAY COOLER. That is a myth, if you can explain that one to me, you could proly win a Nobel Prize in Chemistry.
hope this helped.