It used to be that comic books were printed via thin metal plates (later plastic) adhered to rotating cylinders. If a printer wanted to make any changes to the printed page -- such as blocking out a bar code, for example -- the correction actually had to be taped or glued onto the metal plate prior to printing. Comic books aren't printed that way anymore -- most modern printing is done digitally. Very few original comic book plates still exist, as they were usually melted down after printing so the metal could be re-used. So, they're well worth grabbing, for anyone who happens to run across one. Expect 'em to have a little rust on 'em, though!
Here is a surviving plate for the outside cover (front and back) of Green Lantern #146 (1981), dried ink, rust, and all. Yes, the plate images are always reverse images of the printed pages. The printed inside cover, and all interior pages, also had plates like this:
Here's a scan of the printed cover.