Why must the pH of swimming pool should be kept at their level?

pH determines how effective your chlorine is. The higher your chlorine, the less effective & vise versa.

The 2nd reason is, once the pH drops below 7.0, pool water becomes acidic. The water will eat away at the pool surface, pump & filter.

Hope that helps!! Also, at the other end of the scale . . a pH of 8.0 or higher the water then becomes alkaline and will deposit scale on all surfaces of the pool and equipment. Both high and low pH make the water aggressive. Meaning that the water, chemically speaking, will attack all surfaces of the pool or it's equipment. At above 8.0 the water also feels uncomfortable or may make you itch and will deposit scale which can feel rough to the touch. At 7.0 or lower the water etches and will also feel uncomfortable and make you itch. Both will dry the skin or make it feel dry. Proper pH balance and Total Alkalinity balance will lengthen the life of chlorine somewhat. Balanced water in not only good for the pool but good for the swimmers.