> Despite similar rates of drug use, black people are nearly four times as likely to be arrested for marijuana use,⁴ and black males are often given longer sentences for crimes than their white peers.⁵
[ from https://medium.com/@dan_nott/what-is-mass-incarceration-ff73... ]
Likewise, women tend to make significantly less money for doing the same work, in the same positions, at the same company, because of (largely) subconscious biases.
So called 'affirmative action' movements and diversity pledges are a direct response to the acknowledgement of our unconscious biases. If you know that you're 20% more likely to judge a candidate as unfit just because of the colour of their skin or the sound of their name, then that means that, over time, you're losing out on a lot of talented candidates.
Likewise, if you're less likely to fairly evaluate a woman's performance or contributions, then you're going to promote talent to positions of leadership or management less often, meaning you're not going to put your best people in those positions.
Lastly, diversity is important. You don't get anywhere by looking at only one viewpoint. Look at the state of American media right now: you have tons of content on Fox News that repeats the same few lines as facts, and they've all convinced themselves that it's true because they hear it everywhere. With that environment, why would anyone consider a different fact? When you're surrounded by people who have the same experiences with you, the same viewpoints as you, the same background as you, the same friends as you, you're never going to learn anything.
It's when we surround ourselves with people different to us that we grow and learn. We start to see things from other points of view, we learn about new experiences that we didn't have growing up. We create new collaborations that build on our different backgrounds to produce something that feels new, and not just a retreading of existing works.
In other words, the reason for forcing diversity is to both counter our own (and others') conscious and subconscious biases, as well as to ensure that you're exposing people to as many new and differing viewpoints as possible (and, in turn, taking those viewpoints seriously), so that you have more resources to build from, which makes for a more robust environment.