If U.S. copyright is automatic, then why do people register for U.S. copyright?

U.S. Copyright protection attaches immediately and automatically as soon as you fix the work in question in a tangible form. That tangible form might be a printed manuscript, but computer disks (or even your computer hard drive) can also be considered tangible forms. Registering a copyright with the U.S. Copyright Office creates a public record of the basic information of your book.