Safety: If you are using either a salt based pool producing chlorine or a chlorine based tablet pool, then your pH needs to be correct. If pool water is not balanced correctly then the chlorine sanitiser won’t be working at full strength killing germs and bacteria.
Comfort: Your water balance needs to be correct or it will affect your skin and eyes. The pH should be neither too acidic nor alkaline in order to feel comfortable to swim in.
Corrosion: An imbalanced pool can be corrosive to the liner, ladders and hand rail and other equipment such as the pump.