While both furnish heart-healthy omega-3 polyunsaturated essential fatty acids, cod liver oil also contains considerable amount of vitamin A & D. Vitamin A is especially important for night vision, for fighting against infection (body immunity), and in regulating the genes to make sure they function properly.
Vitamin D is vital for those living in northern regions of the world, (the Northern US and Canada, for example), in the cold months of winter, when people don't get very much sun exposure on their skin. The body generates vitamin D through absorption of sunlight through the skin.
Please note that today's cod liver oil is NOT the same cod liver oil which typically comes to mind from many decades ago. Most cod liver oil from the past was rancid and tasted terrible because it was improperly processed. It is now easy to find flavored cod liver oil and fish oil which have a much more pleasant taste.
Prior to the fortification of milk with vitamins A and D, code liver oil was commonly given to children as a nutritional supplement. As well as being an excellent source of vitamin A and vitamin D, it is also a good source of omega-3 fatty acids (EPA and DHA).