The supplements-aren't-necessary-advice is often the lawerly "you don't need supplements if you're not malnourished" or "if you're getting a balanced diet."

Left unsaid is my takeaway: modern agriculture has damaged our food supply to the point where a nourishing and balanced diet isn't readily available in the produce section of your local supermarket. If you're eating stunted frankenfood, you need to supplement, because you're malnourished from eating an unbalanced diet.