Without getting technical, it is simply an essential fatty acid that the body needs to function at optimal capacity and has broad range of benefits that can range a spectrum of disease processes. They improve cell membrane fluidity, protect the cells by forming part of the cell membrane and control inflammation. As our health is the aggregate of cellular health, we can see that these fatty acids indeed play an important role in our health.
Fish oil is the best source of omega-3s. Your second best bet is Flax seeds. But, two thirds of the population have difficulty converting flax oil to the actual omega-3s that the body needs. Also, it is important to consider the mercury, PCBs and other harmful pollutants in fish oil. Your best bet is wild salmon, as salmon body oils are low in these toxins. Also, it is important to select a product that is tested by independent bodies to be free of these toxins.
Carlson brand of Norwegian Salmon Oil comes close to meeting these exacting requirements of purity as it is routinely tested by independent bodies for impurities and find them to be of good quality.
Here is an article at University of Maryland Medical Center that gives good information on omega-3s which lists it to be useful for diseases like Cancer, Diabetes, Depression, Blood Pressure, Eating Disorders, Skin Disorders, Macular Degeneration etc.