I have heard it used for parasites....the idea is that the water with less salt will rush in to replace the salt water and the parasite will burst.
I heard the fish will lay on itside during this and not look good. I was too chicken to try it.
I haven't heard that much good about this although I am relatively new. I haven't done it myself. When my clown got ich I used copper and it was better in 3 days.
I think it may be used to treat some other fish. May be a really bad idea to an already stressed fish.