Two is that the newly set up aquariums were cycling unbeknownst to the pre-test kit aquarist, and the salt helped to ease the fish's nitrite suffering. Third, the fish, newly arrived, often were with small abrasions due to handling. Fourth is that Columnaris, a bacteria commonly found in such wounds, seems to fare poorly in the presence of salt. It was explained that these developed largely from entering into the fresh rivers and streams while carrying the bacteria and not from the climbs up the falls. Fifth, recently stressed fish are great hosts to hitchhiking parasites.
Salt seems to be an irritant which increases the fish's slime response.
Presumably this helps them to shed and resist parasites. And lastly, getting back to the osmosis gradient, freshwater fish rarely drink due to the constant influx of water to their bodies. Consequently their internal organs must work hard to process this unwanted fluid. Salt would benefit newly imported fish by decreasing this osmotic pressure and reducing the energy expended to merely exist. The above was not intended as an encouragement for anyone to rush home and add salt to their tanks.