Here is the situation:

You need to deploy a new version of your ASP.NET application but the application is running and consequently the .dll's are unable to be deleted because they are "being used by another process". What you would normally do is remote into that machine and kill the ASP.NET worker process (or take the application offline in IIS) so that the .dlls could be released. Only then could you delete the existing files and copy over the new application.

The problem: What if you do not have access to remote into the server and cannot kill the ASP.NET worker process or to take down the application in IIS. How in the world could you deploy these files if you cannot take the application offline in order to release the .dlls from memory?

The solution: ASP.NET 2.0 introduced the feature where you could place a file called app_offline.htm in the root directory of the application, and this would take the app offline. As a result, the runtime files are released and you are able to update your application as well as to provide a .htm page with description of the problem. This is a tremendous help.

Thanks to Sasha for his help finding this feature and if you would like additional information you can check out Scott Guthrie's post on App_offline.htm.