90% of the omega-3 fatty acid in the human brain is docosahexaenoic acid (DHA). It is essential (similar to all vitamins), because the human body cannot produce it itself. More and more research is being done which reveals how vital this compound is for humans. For example, it is known that during pregnancy a disproportionate amount of DHA will travel from the mother across the placenta to the developing baby, indicating its vital role in the development of the fetus.
It is now believed that the evolution of the early human brain, differentiating it from that of its primate relatives at a very fast pace, was due to abundance of seafood in the diet. I.e., early humans were not wandering around the savanna with spears killing rhinos. They were living by the water eating seafood. It is highly unusual for an organism to lack the capacity to manufacture a vital nutrient necessary for its survival on its own unless there exists an abundance of it in its diet.
Sciencedaily.com has an article about a new study published in Behavioral Neuroscience about the importance of DHA in human nervous system function, “New Study Links DHA Type of Omega-3 to Better Nervous-System Function“.