There are many reasons instructors might want to use OER: 

Free and Legal to Use, Improve and Share

• Save time and energy by adapting or revising resources that have already been created
• Tailor educational resources to the specific content for your course
• Expand opportunities for interdisciplinary teaching and learning by allowing you to integrate and revise multiple educational resources
• Redefine "traditional" learning by incorporating multi-media or scenario-based education
• Go beyond the confines of "teaching to the book"

Network and Collaborate with Peers 

• Access educational resources that have already been "peer reviewed" by other experts in your field
• Review or annotation features and texts so other instructors have more in-depth knowledge of the resource and its quality quickly
• Make learning and teaching a team project using collaborative platforms

Lower Educational Cost and Improve Access to Information

• Reduce the cost of course materials, particularly textbooks so that all students have access and aren't as financially burdened
• Find and access information instantly on virtually any topic, on various devices.
• Give learners the option of looking at course content openly before enrolling.
• Reduce the load students bear, possibly increasing graduation and retention rates