Most FDA RDAs are to avoid a state of deficiency. Example: If you don't eat enough Vitamin C, you'll get scurvy. So eat at least this much vitamin C to avoid scurvy -- there's the RDA.

There are a few examples of nutrients which are naturally produced in the body, but supplementation provides a solid benefit beyond what you'd expect from satisfying a deficiency. Vitamin D is a good example of this: I've been taking 10,000IU Vitamin D (~17xRDA) and I feel MUCH better. Creatine is another good one, where you will produce enough to avoid a deficiency state, but additional supplementation significantly improves performance.

The kicker is that everyone is essentially individual in how they respond to things. Some people react really well to a paleo diet (ie high meat, low starch), whereas some people respond very well to a vegan/high-starch diet. Some people don't respond to creatine or fish oil supplementation, and some people are markedly worse off without it. So approach the question methodically: Change one variable at a time and record how you feel.