When you see its licenses the wording will say something like "Creative Commons License: Public domain." This means that the item is not covered by copyright but is in the public domain. So what's Creative Commons got to do with it? Public domain is public domain. It's not something granted by Creative Commons. Yet you see this over and over as if it were!
We, at the Open Clip Art Library are using the Creative Commons Public Domain Dedication for a perfect reason: it gives an explicit definition on what Public Domain is and the consequences of releasing with this license, here is just one paragraph:
Dedicator recognizes that, once placed in the public domain, the Work may be freely reproduced, distributed, transmitted, used, modified, built upon, or otherwise exploited by anyone for any purpose, commercial or non-commercial, and in any way, including by methods that have not yet been invented or conceived.
You will find a lot of materials published on the Internet claimed as Public Domain but with various restrictions imposed, like "no commercial use" or "give credit". This is not Public Domain.
And there is the role of Creative Commons: they precisely define what Public Domain is, what a free-to-copy-but-no-for-commercial-use or a free-to-copy-but-give-credit license is.
Another use of Creative Commons system is that your license is machine-readable and this allowed our metadata format and also the existence of various search engines based on the license.