Some people absorb vitamins, minerals and other nutrients more optimally than others. Simply put, I need my supplements because I don't absorb them well from food.
Due to hypothyroidism, I'm prone to multiple deficiencies (zinc, vitamins A, B12 and D, iron, etc).
I've been deficient in thiamine (vitamin B1) 1 year post-op and required an injection, so I supplement with a vitamin B complex in addition to whatever else I take.
I also had deficiencies in vitamins A and D prior to weight loss surgery, so I take those.
I'm black. Vitamin D deficiency runs rampant in the black community. Our people need triple the sun exposure time to produce enough vitamin D naturally due to darker skin pigmentation, so I'll supplement with it for life.
Blacks also have magnesium deficiency more often than those from other racial/ethnic backgrounds.
The moral of the story is that some of us need supplements to avoid nutritional deficiencies.