Salt is often used to help/aid parasitic infections, for example a salt dip. Some people also add it in low dosages to the aquarium in some cases to relieve stress or to promote healing etc. Personally I would only use it to help if the Goldfish has a parasitic infection as it has a positive affect on most common parasites.
i sprinkle a handfull in the pond once in a blue moon ,i have used it as a treatment as well ,a quick in and out does the fish no harm as with low levels it dont hurt and can be beneficial
I have goldfish and aquarium salt, very little. About 1 tsp per gallon is good for them. I don't constantly use it in my tank because I am afraid I will put in too much with the water changes. I put it in the tank maybe once a month to 6 weeks and it is slowly taken out with my weekly water changes.