Because CC licenses encourage the sharing and reuse of original work, they make scholarship more visible and more impactful. The Creative Commons organization supports the Open Access movement by allowing authors to proactively make their work available for other researchers and the public. However, CC licenses do not negate any publisher agreements that an author may have already signed.
"Open Access (OA) literature is digital, online, free of charge, and free of most copyright and licensing restrictions. What makes it possible is the internet and the consent of the author or copyright-holder." -- Peter Suber, emphasis added
Making scholarship openly available allows others to read it, share it, and build upon it without seeking additional permissions.
Creative Commons licensing is one way that open access literature can be "free of most copyright and licensing restrictions," but there are other forms of open licenses, such as BioMed Central's Copyright and Licensing Agreement. Creative Commons licenses have become the most popular, though.
Most academic publishers continue to hold "All Rights Reserved" for their publications, but Open Access publishers like the Public Library of Science, PeerJ, and Open Humanities Press make the work they publish available with Creative Commons licensing. For more information about open access scholarship, please visit the Directory of Open Access Journals, the Directory of Open Access Books, and the Open Access Directory.