As previously mentioned, it is used to lower the pH in pools as needed. I keep a couple gallons on hand and dump some in when called for. You do NOT want to have your nose over where you're pouring it in and for heaven's sake wear safety glasses in case something splashes back at you! I put maybe 1 to 1-1/2 qts in at a time in a 9500 gallon pool.
Ohhh so it's actually use to clean the water too? I thought was for cleaning the concrete floor of pools and sidewalks etc. I thought chlorine was used for the water. I've never cleaned pools before obviously lol