agree..and then there is the fact that the human body cannot properly digest vitamins. Most of the time the vitmins are excreted intact
For example, I don't get nearly enough sun throughout the year. My vitamin D levels fell to a 5, which can be extremely dangerous long-term.
I started taking D3 supplements and three or four months later I raised my vitamin D levels to a healthy range.
If your body is starved for a particular vitamin, and you take that vitamin as a supplement, the body will be far more likely to extract value from said supplement. Vitamin D for example is among the most common deficiencies, and is very easy to correct via supplements. The reason most vitamins don't make a difference is because most people are not especially deficient in a lot of things.