When there are more than a few replies in a reply thread, having the ability to collapse and expand older replies can be very handy. One no longer has to scroll up and down long pages. Also, this feature implementation has the great side effect of making the ticket page snappier as it reduces the page load time.
When collapsed, it looks like the image below. To expand, you can click on 'See older replies'. To expand further, you have to click on individual replies. However, unanswered customer queries will have the expanded view as default.