The Loader.unloadAndStop feature was added to Action Script 3's API to automate a process that was previously manual when using Loader.unload.
Though Loader.unload removes the child of the Loader object, the unloaded object will still run in the background until it is actually disposed of by the garbage collector. Since the garbage collector does not dispose of objects that are referred to, the "unloaded" content could perhaps never be removed (for example, event listeners could reference the object).
Example: A SWF file that has a music track is loaded into an application. Later, the SWF is unloaded using Loader.unload(). Though the SWF will be removed from the screen, the music will still be heard.