1) Before an infringement suit may be filed in court, registration is necessary for works of U.S. origin.
2) If made before or within 5 years of publication, registration will establish prima facie evidence in court of the validity of the copyright and of the facts stated in the certificate.
3) If registration is made within 3 months after publication of the work or prior to an infringement of the work, statutory damages and attorney's fees will be available to the copyright owner in court actions. Otherwise, only an award of actual damages and profits is available to the copyright owner.
4) Registration allows the owner of the copyright to record the registration with the U. S. Customs Service for protection against the importation of infringing copies.
Expanding on the points one by one.
1) Without a registered copyright, you can't file suit for copyright infringement. That means if somebody infringes on your work and isn't willing to correct the situation to your satisfaction when you complain, you'll have to register the copyright before you show up on their radar as an actual threat. There may be situations where you can take legal action for some other cause related to the theft, but not for copyright infringement.
2) If you don't register within five years of creating the work, the court doesn't have to recognize the registration you do file as self evident proof. Not being a lawyer, I don't know how a smart lawyer would go about getting such a registration overturned, but it's easy to imagine in the Internet age. For example, somebody sent me a long joke this morning, and out of curiosity, I did a quick search to see if I could figure out where it originated. No such luck, it appeared on a large number of blogs and community sites back in 2007 when it spread like lightning. I wouldn't want to be the lawyer for the plaintiff in 2012 trying to prove that the joke originated with my client. And I wouldn't want to the plaintiff paying for that research.
3) The award of statutory damages and attorney's fees is the big stick in copyright law that prevents most cases from going to trial, less than one percent of infringement suits filed. The statutory damages will likely be less of an expense for the infringer than paying the plaintiff's attorney fees if the case goes as far as trial. Figure on five figures just to get the ball rolling, and six figures if the case ends up going to trial. If the copyright isn't registered three months after publication or before an infringement, that's cash you'll have to pay out of pocket, which is likely to be more than any damages you can hope to recover unless the infringing work was a bestseller or had that much of an impact on your own sales. The exception here is if the infringement was online, in which case the Digital Millennium Copyright Act (DMCA) may allow you to recover legal costs, if the court decides to award them.
4) If you've registered the copyright, you can appeal to the Customs service to protect you from illegal imports, perhaps from countries where you have no chance of going after the infringer.
If you need to register a copyright in a hurry so you can launch a legal action to seek an injunction against somebody who is destroying your business, the current fee is $685, and you may need to provide proof of pending litigation or Customs complaint in order to get it. But the bottom line is this. In most instances, a registered copyright will ensure that if somebody does infringe on your work, they'll be under tremendous pressure to remedy the situation once they get legal advice. An unregistered copyright might give them pause, or it might give them a chuckle. Depends on the depth of their pockets and their darkness of their souls.