While registration is not required to have a common law copyright, it is required to be able to sue for copyright infringement in the United States. Copyright registration in the U.S. is not mandatory, rather it is voluntary, but there are several advantages to doing so.
As mentioned, you cannot sue for copyright infringement on a work unless it has been registered. This may seem counterintuitive, because copyright registration is not required, but it is required to even step into court to sue for infringement of the work. Basically, this is a way to encourage people to register, and therefore begin to protect, their copyrights.