Adding salt is no longer generally accepted to have any health benefits. 20 years ago, the general consensus was that some salt had positive health effects for most freshwater fish. However, at the same time, it was also generally accepted that the fewer water changes you could do, the better. Test kits weren't readily available at the time, so an algae breakout was the only indicator that there was something wrong with your water. Therefore, one would be safe to assume that most people operating this way would have nitrate levels that we would now consider excessive. Salt acts to protect fish from nitrate toxicity. No reason now to wonder why people noticed health benefits from salt, if that was the case.
*exceptions to this rule are fish from naturally saline environments, such as many livebearers.
*salt used in fish that are not naturally adapted can cause liver and kidney failure. Many plecostomas are very sensitive, and references going further back like Innes even reference salt causing liver failure.