Why is an extra 'daily intake' of EPA and DHA necessary ?

The body cannot generate EPA and DHA by itself. It needs to derive the materials necessary to produce EPA and DHA from a diet that is rich in omega-3. Unfortunately, a typical Western diet lacks omega-3 fatty acids. That has not always been the case. Historically, we have evolved from a diet that contained approximately as much omega-6 as omega-3 fatty acids. However, our current diet contains approximately twenty times more omega-6 than omega-3 fatty acids. Commonly used vegetable based oils (corn oil, sunflower oil,...) and processed foods consist almost exclusively of omega-6 fatty acids and hardly any omega-3 fatty acids. Therefore, taking EPA and DHA supplements is a conscious choice you can make to achieve a balance between omega-6 and omega-3 fatty acids.