* Registration is also needed to get the longest available copyright term.
* There's no need for the registration to happen anytime near when the work is created or published. If I needed to sue someone for copyright infringement for a piece of software I wrote and published in 1998 and stopped maintaining in 1999, I could file the registration on Tuesday and the lawsuit on Wednesday.
https://fairuse.stanford.edu/overview/faqs/software/
"Even if you don’t put that little © on your work, you automatically get copyright protection the instant your work of expression becomes fixed in a tangible medium. Theoretically, this means that you own the copyright, and no one may copy, distribute, display or make adaptations of the work without your permission."
The difference is it must be registered for the creator to sue for monetary damages.
https://fairuse.stanford.edu/overview/faqs/registration-and-...
"You must register your copyright with the U.S. Copyright Office before you are legally permitted to bring a lawsuit to enforce it."