Canned tuna in water which you're likely already familiar with, is primarily used as an ingredient in tuna salads and casseroles. It's inexpensive, mild in flavor, dry and easy to flake which is honestly all you need for a salad thats going to be mixed with mayo, mustard and other flavorful ingredients.  Oil-packed tuna is more of a specialty "tinned fish" product typically imported from Spain or Portugal. It's ready to serve straight from the container, often filleted and canned by hand, resulting in larger, appealing chunks as well as a higher price tag.