App_offline.htm page allows you to stop all the incoming request to IIS server. If this page is in root of the web application, ASP.net application get shutdown unload the application from server.
All the dynamic pages request send the content of app_offline.htm page back, if it presents in application root directory.
This page you could put when you are uploading a new version which take time to upload and configure it, else end user will see errors on your site.
Once you delete the page from root, new incoming request load the application and app domain, and respond the actual user data.