It is like any other organism, humans adapt to the conditions where they live. An Eskimo does very well in the north of Canada. They can live on the diet that nature provides for them 10 months of the year. They likely would not do well on the Hawaiian Islands. The conditions are quite different.
We need diversity because agricultural exports come from a land that that can best grow or feed them. We are a complex species and have discovered that we need all sorts of things to keep us healthy. A mango or avocado is quite different from a loaf of bread yet all three help to keep us in good shape and all three are grown under entirely different circumstances.
We need diversity because of preference or necessity. We live where we work. Drilling for oil in countries which are harsh environmentally is an example of necessity and we have to overcome that adversity. On the other hand, given how we have "civilized" the environment, we can live anywhere and for me a warm place is preferable to a cold one.
