Why is it so necessary? 

Many dealerships are required to have and use a refractometer to check the "PH" value of engine coolant and battery fluid. For years, electrolysis has been a major concern of heater core and radiator failures. The correct "PH" level of the coolant can eliminate many heater core failures. A refractometer will give the most accurate reading of the coolant "PH" so that proper repairs can be performed.