Using salt is an old "trick", and, as far as I know, there is no real evidence that using salt on a regular basic in freshwater tanks is beneficial. Salt can be used, successfully, to treat some illnesses, but the salt level in the tank has to be higher than normal, because when having salt in the tank all the time, the system gets used to it, in a way. It's supposed to ease osmotic pressure and prevent disease, but it doesn't work like that in long term. Also, there are a lot of fish and plants for which added salt is not recommended.
I read a lot about this subject a few months ago when I was planning to use salt on a regular basis in a livebearer tank. After consulting with several more experienced people, I came to the conclusion that I'll be better off with no salt. You will find different opinions on this, which can be confusing, but at the end of the day it makes sense that salt doesn't belong to a freshwater aquarium on a regular basis, and most probably has little benefit, while it can be harmful to many fish and plants.