Very convenient when you go to the topic and see the selected new comments from the last visit. But a simple example:
We answer in the hot topic, while we see that the mail agent notifies us of new comments in the topic several times. Quiet appends a comment, click send. And what we see? Thanks to AJAX, missing comments are loaded (all?), But they are marked as already read (not marked with a standard rectangle). But it is worth pressing F5, as this rectangle appears.