Postby gwenhyver42 » Mon Feb 25, 2013 9:18 pm
It opens up the reply box at the bottom of the page so that you can post without the delay of the reply page loading. Plus, you can leaf through the posts before it without having to scroll through the tiny view thing at the bottom. If someone else posts while you're typing, it does pop up with, "Are you sure you want to post, or does this new comment change your mind?" (or something along those lines). It's... just awesome.
A much wordier version of what Nik just said...