The fat in fish is rich in polyunsaturated fatty acids called omega –3s. Fattier fish such as salmon, tuna and mackerel are especially good sources since they have more fat. These fatty acids reduce blood clotting and may lower the risk of coronary artery disease and fatal heart attacks. 
They may even be beneficial against things like rheumatoid arthritis and psoriasis and there is some suggestion that they can reduce the risk of macular degeneration (a leading cause of blindness). There is no government recommendation for omega-3 intake, but we all should get as much as we can (and food is the best source, not pills).
The American Heart Association suggests eating at least 2 servings of fish weekly and especially salmon and tuna. By the way, canned tuna is as good a source for omega-3s as fresh. Also, if you choose tuna packed in water rather than oil you’re better off. When you drain the water off you don’t lose any of the omega-3s. Draining the oil however takes ¼ or so of the fish’s omega-3s with it.