Omega-3 in fish and seafood is high in two fatty acids crucial to human health, DHA and EPA. These two fatty acids are pivotal in preventing heart disease, cancer, and many other diseases. Your brain is also highly dependent on DHA -- low DHA levels have been linked to depression, schizophrenia, memory loss, and a higher risk of developing Alzheimer's. 
Researchers are now also linking inadequate intake of these omega-3 fats in pregnant women to premature birth and low birth weight, and to hyperactivity in children.
Omega-3 is also one of the rare instances when taking a supplement instead of getting it from your food is highly recommended. This is because the major source of dietary omega-3 is fish, and most fish that is commercially available (even wild-caught varieties) are polluted with mercury, PCBs and other toxic substances. If you can find a source of lab-verified clean and pure fish, then this would be your best source of omega-3. But for those of you who can’t (the vast majority of people), a supplement is your next best option.