Makes the target movie clip draggable while the movie plays. Only one movie clip can be dragged at a time. After a startDrag() operation is executed, the movie clip remains draggable until it is explicitly stopped by stopDrag() or until a startDrag() action for another movie clip is called.
For example, you might want to create a slider or a scrollbar for a web application - these things can easily be made using startDrag(). Apart from that it can be used in games which actually isn't a novelty since I can remember quite vividly that I saw games employing drag and drop many many years ago (for example a game called "Glasses" where you could drag a pair of glasses that could see through... well, I guess I shouldn't write what;)).