In the context of OER, diversity, equity, and inclusion means including a wide range of perspectives in your learning materials. This can help ensure that more readers identify with and relate to the material. Some benefits are:
BCcampus Open Education Self-Publishing Guide
"The focus of many open education projects is to provide access to education. But what does access mean? If the materials are not accessible for each and every student, do they fulfill the mandate to deliver fully open education? The open education movement has helped people in different parts of the world access content that they would otherwise not be able to view or interact with. Open education resources reduce costs for students and allow for greater flexibility for instructors. Accessibility can help push the movement even further forward."
The University of British Columbia OER Accessibility Toolkit