It helps to distinguish the living animal from the meat; the words "beef" (the meat) and "cow" (the animal) can't be confused because they are different, and in the same way, tuna and tuna fish are used to distinguish the two.
Lol. I think there should be a template for serious answers to Philosoraptor-like questions, in the same way that there is the "actual advice mallard" template for serious advice.