An experienced print operator would use the black lines to set up paper trimming. Traditionally these marks were drawn in the page's graphics stream. This is problematic because nowadays printers prefer to use their own custom marks instead of ones created by the document author. PDF solves this issue by moving these graphics operations to separate draw contexts (specifically "form XObjects", which are not actually forms, though they are XObjects) that can then be "glued" on top of the page. These annotations are shown in PDF viewer applications but they are not printed. I have no experience with high end RIP software, but presumably the operator can choose to either print the document's annotations or replace them with their custom QA marks.