"Just eat healthy and balanced" – this advice ignores our biochemical individuality. Genes significantly determine how efficiently our intestines absorb vitamins, how they are transported in the blood and how they are guided into the cells.
Even with the same sun exposure, some people have chronically low vitamin D levels. Variants in the GC gene or in the VDR (Vitamin D Receptor) are often responsible.
Another example is beta-carotene (from carrots). It is only a precursor. The BCMO1 gene encodes the enzyme that makes real vitamin A from it. In many people, this conversion is significantly reduced.
Genetics is not fate. Knowing your predisposition allows you to get the best out of your biology through a targeted lifestyle.