Aquarium Salt has no Additives such as iodine or calcium silicate. A one-pound box of Aquarium Salt is very inexpensive. It also does not change the hardness. A few Fish are Intolerant of Salt. Corydoras catfish and many live aquatic plants do not do well with salt in their water.
Salt lessens the Number of Pathogens in the water.
For example very few, perhaps only one or two groups, of freshwater bacteria tolerate salt well. My experience is that salt is slightly effective in preventing fish trouble in freshwater aquariums but only slightly. Salt is really most Effective, when the concentration of salt changes. So I keep my freshwater fish in water with very little salt. Then when I see one or more of the Signs of Stress or Disease, such as clamped fins, I immediately add 1 Tablespoon of Aquarium Salt per each 5 gallons of aquarium water.