"OER are teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and re-purposing by others. Open educational resources include full courses, course materials, modules, textbooks, streaming videos, tests, software, and any other tools, materials, or techniques used to support access to knowledge."
There are several reasons why OER's matter from soaring costs of books and journals to timely access to the most current material to general knowledge sharing and learning enhancement. Here are a couple of sites that explain in more detail.
Pros and Cons (University of Maryland University College)