You can add files to your article and this window let's you display the list of those documents. If a document is a mp3 file, you can add a player to the list to make that file playable. This is a simple window that sometimes comes in very handy.
Remember that Calendar module records are very short. If you want a longer version of an event description you can always attach an article to a specific event. It can be a news article from your news feed or just a special article you wrote to make a story about what is gonna happen.