Why use OERs? Because the high cost of textbooks presents a barrier to student success.
In the 2020 US PIRG report, 66% of undergraduate students skipped buying a required textbook because the cost was too high. More than 90% of these students did so even though they believed it could hurt them academically.
A meta-analysis looked at "the efficacy of open textbooks compared with commercial textbooks on learning performance and withdrawal rate. Based on the findings of 22 independent studies, there appeared to be no effect on learning performance in courses with open textbooks compared with courses with commercial textbooks."
The same meta-analysis found that "the withdrawal rate from courses with open textbooks, based on 11 independent studies, was reliably lower than that for courses with commercial textbooks." Source: Efficacy of Open Textbook Adoption on Learning Performance and Course Withdrawal Rates: A Meta-Analysis.
Required textbooks can represent a significant cost to students.