It has no legal effect. Many years back, a cocktail of copyright statements and "All rights reserved" were needed to enforce your rights, but this has not been the case for a long time.
That's not quite correct in the US. A copyright notice may help you in the US if you sue someone for damages for infringing your copyright, by making it harder for them to prove that they are an "innocent infringer".
An "innocent infringer" is someone who did not know and had no reason to believe that they were infringing. If the defendant can prove that they are such (and they bear the burden of proof for this), it greatly reduces the lower bound on the range of damages the court can award.
Including a copyright notice nips that possibility in the bud [1]:
> If a notice of copyright in the form and position specified by this section appears on the published copy or copies to which a defendant in a copyright infringement suit had access, then no weight shall be given to such a defendant’s interposition of a defense based on innocent infringement in mitigation of actual or statutory damages, except as provided in the last sentence of section 504(c)(2).
(The exception is for some non-profits, libraries, public broadcasters, etc., in the case where they thought their use was fair use and turn out to be wrong, and allows reducing damages to $0 for them)
How in the world would the "innocent infringer" defense play out, considering that any and all copyrightable content is implicitly copyrighted at its conception? Especially considering that the declaration does not assist in distinguishing between copyrightable and non-copyrightable aspects within publication.
Saying that the user did not understand that content was copyrightable seems akin to claiming that they did not know stealing was wrong. It is the burden of the individual to know the laws they must follow, and not knowing them is not a valid defense to the best of my knowledge.