SharePoint comes with a boatload of application pages that reside in 14/15 (depending on SharePoint version) hive folder structure.
e.g. In case of SharePoint 2010 you can find them in
|%systemdrive%Common FilesMicrosoft SharedWeb Server Extensions14TEMPLATELAYOUTS|
In this hive folder you will find a bunch of .aspx pages. These pages are used by “Site Settings” etc. You can also create your custom application pages as well by using Visual Studio. Business case for creating custom application page would be you want to create a form that allows end-users / site admins to submit request for creating departmental sub-site or bunch of libraries / lists with custom content types.
Here are some useful OOB application pages along with sample scenario where you can use them.
Signing out the user
Say you want to create a custom link that allows the user to sign out. You can do so by using this url:
Signing in as a different user
Display access denied page
Say you have some code in Visual Studio and the operation you are trying to perform isn’t valid for the current user due to lack of permissions. We can redirect the user to access denied page.
Removing faulty web parts from a web page
When you add web parts to a page sometimes due to unhandled exceptions or configuration issues a web part can cause the whole page to become inaccessible. To remove the faulty web part we can use following application page:
Here /SitePages/Home.aspx the relative url of the page that contains the faulty web part.