Fish vary in their diet. The difference is not great, but tropical fish live in warm water and their physiology is adapted to a particular environment. They like food with ample protein and certain types of fat, and ideally the flakes do not cause unwanted germ growth in water that might be slightly warmer than room temperature. Tropical fish food is formulated to keep certain species healthy as well as simulate the type of food they would like to eat.
An abundance of omega-3 fatty acids is one of the big differences, as tropical fish might be evolved to deal with more fertile waters and its scarcity of some nutrients. The flakes also contain trace amounts of medicine that help to boost immune function and help fish cope with living in a bowl. This does not entirely replace filtration and water circulation, but it is an extra step to preserving your pets.