Essential fatty acids, such as fish oil, and flax seed oil tend to oxidise rather quickly. And when they oxidise, they form free radicals and lipid peroxides, both of which are toxic to nearly all of our cells.
Vitamin E is one of the most powerful antioxidants (with many health benefits), so taking it along with any sort of essential fatty acid -- including fish oil -- helps prevent this oxidation, which allows you to get the most nutritional benefit from the oil. This is especially the case with fish oil which is most unstable.
When purchasing fish oil capsules, look for the ones with added natural vitamin E. (Stay away from synthetic vitamins which are of questionable benefit). While they may be a little more expensive than the budget fish oils, the added Vit E more than makes up for it.
When taking other oils such as flax seed oil, consider taking a capusule of natural vitamin E with it.
By the way; natural vitamin E helps stabilise all fats in circulation, including cholesterol, thus reducing the possibility of harm to arterial walls by rancid cholesterol.