A customized nutrigenomic diet would reflect your body's specific needs. For instance, some people are more prone to experiencing vitamin deficiency than others. For those whose bodies metabolize vitamins perfectly, there may be no need to consume extra servings (in food or pills) of specific vitamins. Once you know your body's needs, it's easier to fulfill them.
A deeper understanding of your genetic makeup would help, too -- certain ethnic groups react more negatively than others do to Western-style foods and the sedentary lifestyle that often accompanies them, for example. The Inuit have, for thousands of years, relied on a high-fat diet to provide the energy needed to go out and hunt down calories while trudging along frozen terrain. But if you have Inuit ancestors and you're a customer-service rep working in a cubicle in Phoenix, your body's tendency to maximize the storage and use of fat won't do you or your waistline much good, and a nutrigenomic analysis could point this out to you.