Where the copies go
Every copy is inserted directly after the page it came from, so duplicating page 3 gives you pages 3 and 3 again, then the original page 4. To place a copy somewhere else, duplicate it here and then drag it into position with Reorder Pages.
Common uses
A blank page repeated for a form, a title page for each section of a packet, or a page you need printed twice. The copies are full pages, not references, so they can be edited separately afterwards.