We only use the grizzly salmon oil as a supplement. This was mainly to boost Maggie's immune system. Last year was the first year since we've had her that she didn't suffer from Masticatory Muscle Myositis which completely locks the jaw. It's an autoimmune disease and she got it at the same time each year - in November. Last year she didn't because we took the advice of @Lindathedogsmaid and switched her over to Fromm and started to add the salmon oil to boost the immume system. However, it's also good for joints and Maggie broke her elbow the first year we had her, we also have a lab with joint problems too. So in the end ALL of our dogs get it. I have to say that all their coats are much shinier and softer with the oil.
I guess it's like humans .. it doesn't matter how carefully you eat and watch your diet. There will be people who are in need of a little extra to supplement their diet because of medical needs, age, youth, or illness.