Sometimes you won't get enough of a particular nutrient. Like magnesium for me. So I take a supplement and I am healthier as a result. Or like Jokaman said, perhaps I'm not in the mood for eating the heart of a cow and that's going to mean that I'm not getting much copper today, I will take a supplement. Whatever you feel you aren't getting enough of you should either find a reliable food source for or supplement with it. Nutrient deficiencies aren't fun.
Stabbing conventional wisdom in its face.
Anyone who wants to talk nutrition should PM me!