What is the App_Data folder used for in Visual Studio?

It's a place to put an embedded database, such as Sql Server Express, Access, or SQLite.

App_Data is essentially a storage point for file-based data stores (as opposed to a SQL server database store for example). Some simple sites make use of it for content stored as XML for example, typically where hosting charges for a DB are expensive.

The intended use of App_data is to store application data for the web process to acess. It should not be viewable by the web and is a place for the web app to store and read data from.

in IIS, highlight the machine, double-click "Request Filtering", open the "Hidden Segments" tab. "App_Data" is listed there as a restricted folder. Yes i know this thread is really old, but this is still applicable.