In phpBB and other traditional forum software you can just jump straight to the last page.
Navigating with PageUp and PageDown in a Discourse thread feels smoother than in a phpBB where you eventually have to hunt for the “next page” link.
It is only a location indicator (which is of course completely useless on infinite scroll pages).
Really, the scroll bar IS a fresh take on the page list, except it's actually useful for navigating the thread and doesn't force readers to deal with the useless concept of numbered pages when such arbitrary divisions are no longer the primary way of navigating the thread. Once you choose to not implement explicit pages (and accept that it's not possible to load the whole thread in one request), the thread scroller is a natural consequence.