Again I am no expert but I justify it like this. You already have the fish out of the tank and in a stressful situation. Odds are it is going to get sick with something. If you dip the fish before you introduce it into the tank you are killing anything it picked up in the store and avoiding bringing the infection into your quarantine tank with the fish. This is of course if its not already in your tank. I have read where other people do this. It does make sense to me that if you can kill it before it even has a chance to get started in your tank....then do it. It will make the treatment that much easier. I believe there is a treatment out there where the fish are moved from tank to tank. This is kind of what I am doing. Only I am attempting to kill the infection before it can get into my water.