The term "copyright protection" is misleading as it implies that a defensive shield has been installed around your work. The reality is that a copyright -- which you get automatically -- gives you offensive rights (what author David Pressman calls a "hunting license"). Even without a registration you can pursue infringers, send a cease and desist letter, or enter into a settlement. You can also sell your copyright, license it, or bequeath it without a registration. But if you want to file a lawsuit in federal court, you must register, a process that costs under $60 if you're not in a hurry (or over $800 if you are). As a preventative formality, however, and because it provides benefits, many copyright owners register their work when it is first published.
Copyright v. patent. One appealing feature of copyrights is that with a copyright you can sue people for infringements that occur before registration. Because patent rights are not automatic (you get rights when your application is approved), you can only sue for patent infringements that occur after registration. (Note, there is a procedure for chasing patent infringements that occur 18 months after a patent application is filed.)
