Until 1989, a published work had to contain a valid copyright notice to receive protection under the copyright laws. But this requirement is no longer in force -- works first published after March 1, 1989 need not include a copyright notice to gain protection under the law.
But even though a copyright notice is not required, it's still important to include one. When a work contains a valid notice, an infringer cannot claim in court that he or she didn't know it was copyrighted. This makes it much easier to win a copyright infringement case and perhaps collect enough damages to make the cost of the case worthwhile. And the very existence of a notice might discourage infringement.
Finally, including a copyright notice may make it easier for a potential infringer to track down a copyright owner and legitimately obtain permission to use the work.