Who's taking fish oil supplements? People who are concerned for their health or people who are generally healthy? Observational studies like this seem much less useful than a dedicated study with properly allocated controls
Fish oil supplements come in a wide range of quality, and dose. And for the same reason that they are nutritionally beneficial, namely they are chemically labile, they also can go rancid readily.
They were prescribed for me when I was getting eye surgery (for example) because the omega-3 fat acids are supposed to improve healing and reduce dryness.
They are also supposed to improve heart health and lower the chance of a heart attack. Edit: sorry, forgot to add that this article blows that up though.
Some people think they are good for brain development. I've met some parents who gave their young kids a few pills every day hoping it'll help make them smarter.