Whilst some sub-systems we are used to using on our computers already hide the file name from us, it is not universal. Sometimes you know you just want the unread mail. Sometimes you can pick the mail from the ones that you saw in the past day or two. Sometimes you want the last one sent by a given person. If you use a mail reader with the mbox filing mechanism the individual emails don't even have a name. If your mail reader uses the maildir system they do have names – but you don't ever need to know them. This is true even if you are using a mail reader from the command line. The emails in scope for the set you have selected are simply numbered and you use the numbers.