"Imagine I took a standard piece of paper. I could fold it into 4 pieces, then cut the top and bottom a bit, staple it, and have a small book. This is called a signature. They can be as small as 4 pages, or much larger. A book is typically made up of several signatures.
The result is, I can take two 4 page signatures and make an 8 page book, but I have no way to make a 9 page book. If I add one page, I have no way to attach it. You can imagine if I stick the page in and just glue the end, it will easily fall out. I might be forced to make it fit in a 7 page book, or maybe print a 12 page book with some blank pages (some print methods can use 2 page signatures).
The short answer is that when making books its usually easiest to make them a certain way, and blank pages may be the result. A children's book might be 30 pages, but the publisher finds that one 32 page signature is the cheapest method of production. So they might add something to the pages, or maybe they leave them blank." (credit Travis83)
"Different reason depending upon if the book is machine or hand bound. I'll mention the handbound reason, which is the original reason for having these blank pages. The opening blank pages are called fly leaves. The pages with writing/art is called the textblock. These pages, if loaded with art (illuminated) sometimes took days to create. The "pages" were vellum (calf skin) and as you can imagine were expensive to make. You want to protect this investment. When books were bound in leather, the tanned leathers would leak and damage the textblock, so the fly leaves were to protect the writing/art from damage. You would use the minimum amount to protect the text block because vellum was expensive to produce. With the advent of fiber paper, you could increase the number of fly leaves. Depending upon on the binding technique used there would a different number of these fly papers. Also, fly leaves are constructed to add structural strength to the book. A book opens and closes and making the hinge strong and durable are important, especially when you consider a town would save up just to buy one book. So there are numerous different construction methods in hand binding that is reflected on the type and number of fly leaves." (credit rtfminc)And here's the Wikipedia page on endpapers (inside covers + flyleaves), which I think will be the subject matter for the embedded images in the next divertimento.