hi memnoch, and welcome!
take some time and introduce yourself in the welcome section and tell us what sort of fish you keep and a little about yourself if you like.
salt is probably the oldest remedy used in the fish keeping hobby. it helps build up the slime coat that fish have by slightly agitating the glands under their skin. this slime coat keeps diseases at bay. - possibly the simplest reason, though there are many many more reasons that are a million times more complicated and techincal.
so i use it because of that and because its just a habit now lol.