Antifreeze (also known as engine coolant) has three major functions. The first of course is to cool the engine down. Without antifreeze your engine will very quickly suffer catastrophic damage. The second function of antifreeze is to keep the coolant in the engine from freezing (It's like the function is right in the name! Weird right?). If the coolant were to freeze inside the engine it would expand and crack the engine on the inside. The third function and one of the more important ones in our tropical climate is corrosion protection.
Degraded coolant will negatively affect all three major functions of the coolant. Whenever antifreeze gets old it will break down and form solids in the engine. This looks like either murky coolant or gunk buildup inside the radiator. These solids will do massive amounts of damage to the engine seals and moving parts like the water pump.
A coolant power flush will remove more of the old coolant and solids inside the radiator than simply draining and refilling the radiator will. It first uses a special chemical to remove any buildup from inside the engine walls and radiator passages. The flow of coolant is then interrupted which pumps new coolant into the engine while removing old coolant from the other side. A conditioner is then added to help protect the various seals that keep the coolant inside the engine where it is supposed to be.