Traditional blood testing shows how much of a nutrient is in the blood, referred to as the "blood level" of said nutrient.
However, many factors influence how a nutrient actually functions in a body. While knowing your blood levels is better than nothing, it only reveals how you compare to the population standards, not how the nutrient is working in your body. Your optimal range for each nutrient could be considerably higher or lower than the norm.
Metabolite testing takes a personalized approach and shows you the dose required based on what is going on within your body, not based on population studies that you were not a part of.