It's a remnant of the Buenos Aires convention. Where if you didn't expressly reserve copyright, the work defaulted to public domain. Because all the countries

It's a remnant of the Buenos Aires convention. Where if you didn't expressly reserve copyright, the work defaulted to public domain. Because all the countries who signed that treaty didn't move to the Berne Convention right away, you had to still reserve your copyright or else lose it in those countries. The last holdout signed the Berne Convention in 2000, so it's no longer an issue.