Drag and drop let you simplify the needed web procedures. For example a user doesn't need to choose a file for uploading, it can drag and drop it on the web page; or he can manage the page view by by dragging its elements from one place to another (fro example the menus). On the other hand it is easy to learn and use. Please see:
Drag and Drop HTML elements
Drag and Drop HTML codes
Drag and Drop Files
Drag and Drop Events and Styles I and II

and the following image is an example of generating files information when they are dropped on the page: