Because aquarium salt soothes fish. If they are stressed (from shipment and crowding) they over do it to relax the fish and just take it too far in my opinion. Some fish are intolerant to salt and it cuts their life down. Plecos, Corydoras and other salt intolerant fish should never be purchased from Petco or Petsmart because of that.

As for them recommending it, they just wanna make money. In some cases with certain species, Aquarium salt is really important.

because it is a general tonic and also helps boost fish immune systems. however some species of fish (most tetras in particular) do not like salt and salt will hurt them. only keep salt in tanks with salt tolerant fish. i have salt in my aquarium with all livebearers and my molly needs it.

They want to sell more salt?

It is beneficial for some fish, like Mollies, but is toxic to many others.

Ian

aquarium salt improves gill function.

You should never use salt in a freshwater tank without a good reason. If helps with some illnesses and reduces stress in some types of freshwater fish. Big box stores that sell fish use salt in the water because the fish are usually in poor health to begin with. Many of these fish actually have no tolerance to salt though such as corys and scaleless fish. The main thing to consider in box stores is that all the tanks share the same water. some fish need a little salt such as mollys and some need it because they are disease infected. The rest will get the salt because it is in the water. None of these fish are expected to be in the tank for long.