Because the wet towel will form a layer of ice around the fish, which protects the fins from freezer burn. I've seen a lot of fins crumble into nothing because of improper freezing. Then the plastic bag will prevent the fish from freeze drying, because most frost free freezers will pull the moisture right out of your fish. Creating a bad case of freezer burn. So wet your towel good, wrap the fish in it, place it in a good plastic bag. Then duct tape it closed. The fish should last for years this way.