Hi zain611,
I disagree with the others who have posted here. API aquarium salt is an inexpensive and effective product to use in the treatment of ick, fungus and bacterial infections. True, there are other medications, such as antibiotics, that are more effective, but salt is easy to keep at hand and so treatment can be started as soon as the problems are spotted.
However, under no circumstances would I recommend keeping a level of salt in a normal, healthy freshwater tank. It's unnecessary and can actually be harmful to some fish.