in reply to Marking replies to updated nodes

The problem it addresses is when an author updates a node with replies such that the replies are invalid or don't make sense. This way, at least readers would be warned that they should think before assuming that the replier is an idiot, and perhaps wonder instead about the original poster.
One way to CYA is to include the text you are replying to in indented italics. This serves to document what you were commenting on. Could be annoying if overdone, but I thought I'd mention it here anyway.