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Unlike Public Domain, the WTFPL is actually a license, and confers rights to the user of the software as such, whereas Public Domain doesn't confer any rights, but is merely an acknowledgement that no rights are asserted. That's why a license is always preferable to Public Domain; a license explicitly gives permission.
The public domain is a bit hard to define formally, as it varies per jurisdiction and there are even jurisdictions that don't recognize a creator's right to dedicate her works to the public domain. This is exactly the problem CC0 is designed to solve:
CC0 is a license that is equivalent to dedicating your works to the public domain, but it also takes care of the various legal issues and ambiguities of the public domain by simply being a license. The same, more or less, holds true for WFTPL, it's an extremely permissive license you can apply to your works and not worry about the nuances of the public domain.