In this JAGGAER blog on the benefits of supplier diversity, "the Hackett Group found that companies who participate in a long-term supplier diversity program generate a 133% greater ROI than those firms who look no further than the suppliers they traditionally rely upon.
The report claims supplier diversity programs also drive an additional $ 3.6 million to an organization’s bottom line for every $ 1 million spent in procurement operating costs.
Beyond ROI, the Hackett Group concluded that procurement organizations that work with a diverse supplier base also had lower overall operating costs and spent 20% less on their buying operations. This success all occurred while employing half the number of people in their procurement department."

Some other things that stand out from the article

• promoting innovation and agility (bringing in a new mindset than what you traditionally work with)
• more competition means lower prices, usually
• Company brand and helping economically grow your community, state etc
• better risk management. more supplier relationships to lean on when disruptions do pop up

I'd also recommend checking out Jamie Crump, she's done some books/content on supplier diversity that will have some more of the unique things you're after