theres a big sign in my petsmart here in town "All freshwater fish need salt to help with their gill function and to help with stress".
Crazy huh? So considering most people don't just toss the newly bought fish into their tank, it's no wonder most chain stores have bad reps about fish dying. Makes me wonder if all the big chain stores have salt in their water. Even one of the local mom and pop shops tell me they add water to all their tanks for "proper gill function". I stopped buying from that store after they told me that.
salt can help gill function - the chlorine in salt will bind with blood cells easier than nitrite will, and so if you have a nitrite spike, you can help the fish survive it by adding salt. however, that use of it really just applies in large ponds where it is not feasible to do frequent large water changes. in our own tanks, the water changes are a safer, cheaper, and more effective option.
my assumption in regards to stores that use salt in all their tanks is that without it, fish were dying because of poor water quality. i don't think the salt will help them live the same as if the water was clean/safe, but it probably helps them live long enough to be sold, which is what matters to the stores.
as far as helping with slime coat/mucous production (noted in another post) salt does this by irritating the fish. the irritation increases mucous production, and a thicker mucous coat can help to prevent skin-related diseases. the problem is that the constant irritation to the fish over time causes physical/immune stress and can lead to death.
think of salt as the pressure that would cause a callous - if you have repetitive pressure on some part of your body, your skin will form the protective layer. if you constantly are exposed to this pressure/friction to the point where your entire body is affected, that is going to stress your body because your body is always trying to overcome the irritation.