Just one more way to make it easier for users to discover content. There's always people who will tend to focus on the first couple of pages of threads in a parent node/forum, often ignoring content in subnodes/subforums, and this helps ensure that at least the more recent of that content will get more attention. Even better if used in conjunction with Xon's prefix addons. Easy and quick to set up on a per node basis or site-wide.
I wish I knew this existed on release.. Only just bought and implemented this yesterday. I initially suspected a bug, but this proved to be a conflict with a couple of other add-ons which were at fault. Xon very kindly helped with the identification of this.