I had always been taught that supplementation should support foundational nutrition. For me that means:
1. A good multivitamin 2. Cod liver oil and/or fish oils
If extending beyond that I'd say that you can't go wrong with:
3. A good probiotic formula 4. Some digestive enzymes
Lastly I've heard people swear by their:
5. Green food powders 6. Whole food fiber products
Some argue the need for supplemenation is based on nutrient deficient foods that come from commercial farming or obvious gaps in the Standard American Diet.