In my line of work, I've found that by the time symptoms/illnesses arise, the assimilation/digestive processes are so weak that the patient can be eating the finest foods in the world and never getting the nutritional value out of the food.
In other words, I've seen people trying to get well (after the fact) by eating the purest organic foods obtainable, yet they remain sick. But the value in herbology is that the nutrients are stronger and more directed to the needed areas of weakness, more so than anything our daily diets can provide.