Quote Originally Posted by John Koehrer View Post
This was the original intent. The increased coverage of the Nikon F series camera allowed for much more accurate framing for scientific, technical and copy work.
As benjiboy noted it's to allow for that bit of masking from the slide mount AND from a standard negative carrier.
When I had a Nikon F2 and shot mainly slides and mounted them for projection, I found the 100% viewfinder very annoying, I know 100% coverage in theory is ideal , but in practice it was a pain in the ass.