Omega-3 fatty acids are key for normal development and growth. Lack of dietary intake of omega-3 fatty acids and too much intake of omega-6 fatty acids is believed to be a significant contributing factor to many diseases. Since the human body cannot make omega-3 fatty acids, we have to get them from our diet. 
Omega-3 supplements have become one of the most popular supplements in the U.S. They're used to help prevent and treat a number of serious diseases.

Why do people take omega-3 fatty acids? 
Studies show that omega-3 fatty acids from fish oil is associated with a decreased risk of high blood pressure and triglyceride levels. There's also some evidence that diets high in omega-3 fatty acids help with rheumatoid arthritis. Past evidence pointed to omega-3 fatty acids reducing risk of heart attacks, strokes, and death from heart disease, but recent research has refuted some of these findings. More specific research is needed to sort this out.
Omega-3 fatty acids have been studied in many other conditions. They include asthma, cancer, depression, psoriasis, and inflammatory bowel diseases such as Crohn's disease and ulcerative colitis.