[smiley1437]

Some pools use a electrolysis system to generate chlorine for the pool. They'are marketed as salt-water chlorinators.

It splits the sodium chloride in the water into chlorine and sodium hydroxide so you don't have to keep buying solid chlorine tablets.

For the electrolysis to work, it needs bout 3000ppm to 5000ppm of salt, for a pool this could be hundreds of pounds of salt.

As an aside, the ocean is about 35000 ppm salt so this is about 1/10th the saltiness of the ocean.