The study of nutrition and which vitamins, minerals and phyto-chemicals are essential or important is still quite young. A major reason to eat a wide variety is to obtain the important nutrients we don't yet know.
There is some evidence that the food we eat is less nutritious than it once was before mass agriculture. Our soil is depleted and our fruits and veggies are perhaps lacking nutrients. I supplement my diet with a high quality multi (mega food because it's not made with isolates) and things like fish oil, probiotics, and other things if I get sick.
There are non-vitamin and non-mineral compounds in those foods that detoxify/cleanse our body in various ways. They might be free radical scavengers or increase glutathione production or bind to heavy metals. This is where plants belong in the human diet - as foods that cleanse us. Animal foods withotu exception are more nutritious and easily absorbed.