If you’re printing something that’s eventually to be reproduced in significant quantities, and bound, it’s conventional to print on paper larger than your target product, and to place “crop marks” outside the printed area. These crop marks are available to the production house, for lining up reproduction and trimming machines.
You can save yourself the (considerable) trouble of programming these
marks for yourself by using the package
crop, which has
facilities to satisfy any conceivable production house. Users of the
memoir class don’t need the package, since
its own facilities for programming crop marks.