If you publish a report to an App, each report page has a unique URL. If you later on mark that page as invisible and then update the app, that page is still accessable via that URL. This is a problem due to users frequently using Browser Bookmarks, even when requested not to.
The use of hidden pages is important with template and expansion of reporting. We have had users get access to incorrect information due to the use of bookmarking. Users should never be allowed to view hidden. Access to hidden pages should either produce an error, or redirect the user to the first visible page on the report.