In the Flat mode when you click on the lead post you will see all of the following replies with their entire text. In the Tree mode, you see the lead post and you see only the subject line of the replies and you have to click on the subject line header to read the text of those replies. So, as others have already mentioned, the EOM means there's no need to click on the subject header as there's nothing more to read. It's just a matter of preference. I prefer the Tree mode so I use EOM often.