In many countries you can not "release" works as public domain. E.g. here in Germany, a work can become public domain by expiring copyright, but creators have rights that can not be removed (not even voluntarily) before that.
(In such countries, a court might recognize the intent behind a claim of public domain from a country where it is possible, but that requires interpretation of the law of the country the creator is from. And it's likely not an option for locals, since the concept does not exist in local law)
Creative Commons created the CC0 license to work around this: it clearly lays out that a work is intended to be released as public domain, and failing that, all possible rights are granted and the creator doesn't intend to limit them in any way with his remaining ones.