Coolant needs to be exchanged because it breaks down over the years and then loses the ability to serve its purpose of maintaining optimum temperature of the engine and preventing corrosion. Dirt and rust particles are floating inside the coolant which can clog your radiator or heater core. Clogs in the radiator will cause your car to overheat. A clog in the heater core will prevent heat from getting into the car.
In a coolant or radiator exchange, all of the old coolant is drained or flushed out and new clean efficient coolant is added in.