I will NEVER skip my morning omegas!!!! Not only are they good for the reasons that Jewelieta mentioned, but they can also help with depression and mood disorders. If you are of childbearing age, they can help ease post-pardum depression and they help with fetus brain development. Also, when you look at the healthiest peoples of the world, their diets tend to be high in omega-3's.
As far as magnesium, most people in the US do not get enough in their diets. I pulled some info from the National institute of Health website:
Magnesium is the fourth most abundant mineral in the body and is essential to good health. Approximately 50% of total body magnesium is found in bone. The other half is found predominantly inside cells of body tissues and organs. Only 1% of magnesium is found in blood, but the body works very hard to keep blood levels of magnesium constant.
Magnesium is needed for more than 300 biochemical reactions in the body. It helps maintain normal muscle and nerve function, keeps heart rhythm steady, supports a healthy immune system, and keeps bones strong. Magnesium also helps regulate blood sugar levels, promotes normal blood pressure, and is known to be involved in energy metabolism and protein synthesis. There is an increased interest in the role of magnesium in preventing and managing disorders such as hypertension, cardiovascular disease, and diabetes.