Everyone has seen pages like this, typically on reports from financial institutions or health-care firms. It seems to be a cover-your-ass adaptation to the fear that some recipients will be distressed to confront a blank page, or that a malefactor will insert extraneous material in a report. Perhaps some reader can offer insight into the rationale.
Addendum: within minutes of my posting this, a reader provided the link for a Wikipedia entry on Intentionally blank page.
Intentionally blank pages are usually the result of printing conventions and techniques (allow chapters to start on odd-numbered pages, allow for additions later etc). In standardized tests it prevents subjects from proceeding to subsequent sections. Other considerations for sheet music. Most of these examples don't justify the addition of a sentence to declare blankness. Maybe the part re classified document page checks is the best in that regard.
If you've read this far in the post, then it will be worth a couple seconds of your time to peek at the This Page Intentionally Left Blank Project.