Copyfree is not quite the public domain.
The legal concept of the public domain is to copyright as the mathematical concept of zero is to numbers. In the words of the US Copyright Office - Definitions (FAQ):
Where is the public domain?
The public domain is not a place. A work of authorship is in the "public domain" if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner.
A few license characteristics that some may find valuable enough to choose a copyfree license rather than the public domain. Among them are:
Explicit Attribution Requirements: Several copyfree licenses explicitly require attribution for the work to be maintained with the work, in the license, or otherwise, so that recipients will necessarily have the ability to learn who contributed to the creation of the work.
Freedom From Patent Protections: Some copyfree licenses may specify that any patents held by a contributor to a licensed work, that pertain to that work, shall be subject to a royalty-free grant of license for purposes of that work.
Limited Applicability: Many copyfree licenses are particular to a specific medium or class of work, such as licenses that are particular to software.
Popularity: Some copyfree licenses are very well-known, and using the name of such a license to refer to the terms under which a work is available serves as a convenient shorthand for the more lengthy explanation of what the license terms mean for those who are familiar with that license.
If these were the only concerns related to using a license rather than the public domain, however, copyfree.org would never have been created. Two key reasons to choose a copyfree license over the public domain are:
Jurisdictional Applicability: In some jurisdictions, either there is no legal concept of the public domain or one cannot release a work into the public domain before its copyright term has expired. In these circumstances, one must explicitly give permission for others to use a work for any exception to standard copyright protections to apply -- in short, one must use a license to relinquish claim on any license terms. As a result, if someone in the United States releases something into the public domain, it can be used without restriction in the US, but in a country such as France it may still be illegal to use it without getting explicit permission from the copyright holder. Copyfree licenses serve the purpose of broadening the reach of the relinquishment of copyright protections.
Third Party Copyright: When a work is in the public domain, a third party may make modifications to it and claim copyright protections for the modified work. Many copyfree licenses ensure that anything released under their terms, including modified versions of the works, will continue to be subject to the terms of the license, rather than having copyright restrictions applied to a modified version later on.