There were two typical reasons for this happening:
- to emphasise the handmade paper
- to reduce paper and production costs by eliminating a cut and using more of the paper
So it's usually the very expensive or very cheap books which have this.
I've actually purchased a couple of old books which were printed on handmade paper and which hadn't been slit fully, so I knew that I was the first person to actually open and fully read the book.
There was one instance of a famous author finding a a very old book which he needed for research at a library, finding many sections still unopenable 'cause they had not yet been cut, asked the librarian when checking out, ``Who did you purchase this book for?''
--- the librarian's response, ``For you <insert famous author name here>.''
Anyone know the author in question?
Found a great (different) story on this though:
http://mintaka.sdsu.edu/GF/bibliog/library/cut.html
A warning --- if you find a rare book which is unopened, consider / research the value which will be lost by cutting the edges, as well as the possibility of damaging the pages.