Saves Money: Improperly balanced pool & spa water corrodes metal fixtures, fades and weakens liners, destroys concrete/gunite, and worse yet it can bleach (ruin) your favorite swimsuit. Additionally, having to correct a water problem will require more chemicals than keeping the water balanced in the first place.
Aesthetic Reasons: Improperly balanced pool & spa water can look bad and smell bad. No one wants to swim in nasty looking green or black pool water that smells like a swamp. Unsightly stains on pool surfaces caused by metals or algae don't really make someone want to go for a swim, either.