WebDragDropExtender allows you to handle DragDrop event in two ways. Firstly, you can handle the OnDragDrop client side event if you only wanted to perform client-side tasks such as adding or changing an html element dynamically. However in most Web application, oftentimes you need to perform database operations or other server-side related tasks. For instance, you might want to save the modified data to physical database and so on.
With AutoPostback feature, WebDragDropExtender makes it easy for you to handle DragDrop event in server-side. You can also change the Postback mode to FlyPostBack in order to perform the Postback in AJAX way.
To learn more about AJAX and Auto Postback feature, please refer to AJAX DragDrop Server Side Event.