Diversity for its own sake can increase group creativity
One reason we want to have an eye towards diversity is fairness: There are people who might be overlooked as a result of unconscious prejudices. Being diversity conscious helps us to ensure that ethnicity and gender are NOT considerations when choosing to hire someone. For instance, some aspects of a person's background can be evaluated by people who are looking at a resume, completely unaware of a person's race.
However, there's another good reason to consciously encourage diversity: We want to avoid creating a monoculture. Let's say there are open positions for some engineering team, and by coincidence, all the top candidates are all from Elmira, NY. Individually, they are the most qualified. But that doesn't mean they will make the most competitive team. Coming all from the same background, there are alternatives to ideas they grew up with that they are all unaware of. This will reduce the creative potential of the team. By including less (but sufficiently) qualified people on the team that are from more diverse backgrounds, they will bring to the table a greater diversity of ideas (and some fundamentally different ways of thinking) that lead to better and more creative solutions.