Think about the last time you walked down a city street. What did you see?
You probably saw a diverse group of people interacting with one another. Different races and genders are working behind the cash register, on a construction crew for the road work being done, or maybe even a homeless man asking for assistance. Not only will you experience these differences in the city, but you may also see someone with a physical or mental disability.
The world is a very diverse place. Yet, when we go into the office, we may not see a diverse group of people there. Often, the office looks a lot like the person doing the hiring or leading the company.
That hurts not only the excluded people but the organization as a whole.