"Open Educational Resources (OER) are teaching, learning, and research resources released under an open license that permits their free use and repurposing by others. OER can be textbooks, full courses, lesson plans, videos, tests, software or any other tool, material or technique that supports access to knowledge." - SPARC Open Education Fact Sheet
Textbook and other course material costs can have a large impact on the success of that student if they are forced to find alternatives on their own. What happens when students can't afford the course materials?
By adopting OER for your course, you immediately remove one possible barrier to many students achieving their educational goals. Making your course materials free and easily accessible to your students can take away a common stumbling block students face.