Why are essential fatty acids essential?

I'm in the midst of listening to The Perricone Prescription and was struck by Dr. Perricone's emphasis on eating fish. Dr. Kenneth Cooper also strongly recommends fish in the diet. Both of them do it for the same reason, though their approach to it differs significantly. Perricone comes at it from a cosmetic viewpoint ("eating fish will make you look better") while for Cooper, it's a health spin ("fish oils are significant contributors to health"). In the end, the message is the same.

Why is it that some oils are called essential fatty acids (EFA's)? The answer is really quite simple, they are necessary to our body's good function, but our body does not make them. This means it is essential that we obtain them from our diet.

There are many further nutritional nuances, but this one seems to now be coming from many sources. Given the opportunity, eat more fish.