Well I don't know for sure but here are couple of reasons I can think of. 1. to make a new chapter/section always start on a facing page. 2. I used to work f

Well I don't know for sure but here are couple of reasons I can think of. 1. to make a new chapter/section always start on a facing page. 2. I used to work for a banking software firm and they provided very large paper manuals (several thousand pages) to their clients for the various systems. They would always leave blank pages scattered through the manual so they could ship paper corrections to the manuals without reprinting the whole thing. This allowed the page references in the later pages of the manual to remain correct. (This was 20 odd years ago, I'm sure their manuals are PDF now.)