Many books are (or perhaps were, I haven't kept up) printed on large sheets of paper that get folded into groups of 8 or 16 pages (called signatures) and then cut at the edges. (If you look carefully at the spine of a hardcover book you can often see the groups.) Supposing you write a book that's 250 pages long, that means you will end up with 6 blank pages somewhere because 250 isn't evenly divisible by 8 or 16. This seems inefficient with the book in your hands, but printing and folding the large sheets of paper is much more efficient for certain printing technologies so it's an overall win.

Illustration taken from Joumana Medlej's nice explanation of various printing techniques: http://www.makingcomics.com/2011...



Before everything was done digitally, some books were updated by just changing the content of particular pages. For example, suppose you have a 500-page, 20-chapter manual for the maintenance of some machine. If you update chapter 4 and make it a couple pages shorter, you don't really want to go through and re-typeset chapters 5-20 just to fix the page numbers. But leaving a gap in page numbers causes confusion, so you just throw in a couple of "intentionally left blank" pages with the correct numbers. (If you added pages, you'd do something tricky like adding letters. E.g., between pages 20 and 21 are pages 20a, 20b, 20c, 20d.)

A variant of the former is physical books that were updated piecemeal rather than being reprinted on each change. I worked at a library in the mid-80s and there was some government reference book (the Code of Federal Regulations perhaps?) that we would get regular updates for. It was kept in something akin to the three-ring-binder of your school days. We'd get a small packet of pages which we'd insert or replace existing pages with. A freshly printed copy would look insane because of the number of pseudo-blank pages (from deletions) or kooky page numbers (from insertions).



There are three non-aesthetic reasons I know of for blank pages, all related to peculiarities in the technology of printing:They are marked "intentionally left blank", of course, because they don't want readers to worry that a printing mistake has left them missing something good.I have no evidence for it, but I suspect that pages for "reader notes" were almost always just a way of intentionally leaving blank pages in a way that looks less weird to readers. I would also bet that publisher ads in the backs of some books are similar "we've got the pages so let's use them" filler.