In the event that I was not clear, the thread sort would basically mimic a long e-mails chain. The original email is on the bottom and each new reply is appended to the top.
As it is now, if I wanted to copy and paste a relevant but separate e-mail chain into a thread update/reply, the order of the thread would become a bit confusing. You'd go from the original ticket on top, subsequent replies to it as you went down the page, and then you'd hit a reply with an e-mail chain that has the most recent information on top, and not as the bottom as you would expect with the thread sort. Having the ability to choose most-recent-on-top sorting for ticket threads would make copying and pasting relevant e-mails and other correspondence that follows a similar pattern easier and less confusing.