Why is "diversity" automatically seen as being a "good thing"?

I mean, I can understand why biodiversity is important, but I'm speaking generally here. Would the world really end if everyone were basically the same, or at least similar in all the major attributes (like height, weight, intelligence)?

Update: A few people have mentioned the importance of genetic diversity, and yes, I agree with that, I agree that a case can be made for its importance, but I was thinking more along the lines of diversity in general, as a concept. People seem to just lazily assume it to be a "good thing", and very few seem to question whether, for example (and this is just an example), a diverse workplace is inherently good. Why is that? Why the unquestioning acceptance of an idea that hasn't really even been defined properly?