For example on when to take these supplements, let's say you need omega 3 fatty acid to lower your triglyceride levels and blood pressure, or you'd take it to prevent a cardiovascular disease. So, if you would do a bit of research, you'd find that the omega 3 fatty acid can mostly be found in seafoods.
Let's say you're not a fan of eating seafoods - maybe you're vegetarian, or you're allergic to seafoods so you cannot take any. Now, this is when an omega 3 pill can come to be an acceptable and good alternative source.
Let's see some other cases, when you may take a supplement:
Stressy work/ lifestyle: Multivitamins
Smoking: Vitamin C and -E
Workout: Antioxidants, Multivitamins.
Please remember, that there's a cure and alternative medicines for most illnesses and diseases out there, provided by Mother Nature. Fruits, vegetables and herbs can improve your health greatly.
Vitamin Supplements phto: wiki
Rosemary herb photo: wiki