Magic Buttons in Outlook '07

I just noticed these two buttons when reading some emails in outlook, they magically appear on screen when you hover over the area where they are at! If you have a conversation with several emails going back an forth, you can go up and down based on the one being read, to see those buttons hover over the right corner side around the separator line between emails. I’ve only taken a screenshot of one direction, they also show ‘previous’ …etc

Interesting idea, but why aren’t these always visible?