Whats the purpose of the salt for platys in a fresh water aquarium?

Salt is commonly added to livebearer tanks because many of the livebearer species were originally collected/discovered in slightly salty water.

However, platys are freshwater and don't like salt in the water. Mollies like salt in the water. That's one of the reasons they shouldn't be kept together. (The salt softens the water, to a degree.)