This is the quickest and easiest way to display your "Site Temporarily Unavailable" notice while re-deploying (or rolling back) changes to a Production Live server (and you want to avoid the annoying problem of people hitting and activating your site in the middle of a content update).
It can also be a useful way to immediately unlock and unload a SQL Express or Access database whose .mdf or .mdb data files are residing in the /App_Data directory. Once you remove the app_offline.htm file, the next request into the application will cause ASP.NET to load the application and app-domain again, and life will continue along as normal. Lets look at some example. I have created this simple App_offline.htm file.
The application is down for maintenance. Please try again in a while.
<!-- Add additional content to make this file bigger than 512 bytes... -->