That depends on what kind of acid we are talking about. Muriatic (liquid) acid or Dry acid (powder), Cyanuric acid, or Ascorbic acid.
Muriatic (hydrochloric acid) or Dry Acid (sodium bisulfate) is used for lowering pH and alkalinity
Cyanuric acid liquid or granule is your chlorine’s sunblock. It keeps your chlorine from burning off in the sun as fast.
Ascorbic acid or concentrated vitamin C is great for removing metal stains.