I wish I could take credit for this realization but I can't--it goes to my teacher in History of the Book (actually, right now I can't remember the exactly title of that class, mainly because I was sitting in the entire time and wasn't officially enrolled in it: people should try that out some time).
Doing layout for any kind of publication is a difficult task. In magazines for example, you have to be precise. When was the last time you opened a magazine and saw a blank page? Books, on the other hand, have more leeway. Have you ever wondered why at the end of a book, there are some pages that are blank, or worse, publishers try to cover up these "blank pages" by labeling "notes" above those supposed blank pages.
The answer is simple. If you're familiar with the publishing process, the page count of most books is usually divisible by four. The best way to illustrate why it's usually divisible by four is to grab a piece of bond paper and fold it in the middle. You instantly have four pages! Publishing for the most part involves several layers of4-page signatures (sometimes it's eight, twelve, or even sixteen). Depending on the word count and the layout, you might not have enough space to fill up all those pages. Hence the blank pages yet you similarly can't do away with them because the physical integrity of the book will sometimes suffer without those pages.
Of course books aren't as ad intensive as magazines, so figuring out what to do with those extra pages can be tricky. If it's just a few pages, the publisher might run an in-house ad promoting their selection of books. Others might show a preview or excerpt of the sequel.
If you were the publisher, what would you put on those "blank" pages?