chris
No longer a newbie, moving up!
It is probably to show the image cropped to the 4x4 cm format which would allow a slide to fit in the appropriate 5x5 cm holder and so be projected in a 'standard' 35 mm projector rather than use a 7x7 cm slide holder and a bigger, usually more expensive, and often manual projector needed for the full 6x6 frame.