Most likely reasons are to fix some broken HTML, to make a link clickable, to insert white space (blank lines between paragraphs), to make light-colored text black, or to shorten a long link that was causing side-scrolling. Generally, your actual wording is not changed. There is an exception: spam messages are removed by editing out the entire message (or deleting it, if possible). If the actual text of your post is changed, there will usually be a small note added at the bottom of your post (before your sig, if any) saying exactly what was done. If your message is the first message in a thread and there is no such note, chances are your message was not edited at all but that the thread was renamed or moved to a different folder.