When modifying one of your messages, you'll see a new text box towards the bottom which will ask you why you are editing your message. This is useful to update readers as to changes you've made, such as adding new details, correcting a serious error, or just rewording something in a message to make it easier to follow. You aren't required to enter anything, but it's there in case you need it.
Please note you have a five minute window to edit your post without it being labeled as edited. If you change something during the first five minutes after your original post, the reason for edit feature will not work.