Halibut liver oil is an excellent source of preformed vitamins A and D and is easier for the body to use than beta carotene for some people with digestive disorders. Vitamin A is a fat-soluble vitamin that promotes healing and helps to boost the immune system. It benefits the eyes as well as soft tissue.

Vitamin D, the sunshine vitamin, is also fat-soluble and acts as a hormone to assist the body's absorption and utilization of calcium. It helps bones and teeth, stabilizes the nervous system and helps the blood and cardiovascular system and muscles. 

Although fish liver oils are excellent sources of preformed vitamin A, they should not be relied on as a source of essential fatty acids. You would consume too much vitamin A and D before getting what your body requires in omega oils.