Thanks for posting that; I'd never seen it before. As my sister comment said, for smaller subs it could be quite useful for finding active discussions - like on /r/truefilm or something.

People always say "sort Reddit threads by comment age if you want the 4chan experience" and that has some merit - but that only applies once you have chosen a thread to open. A subreddit-wide comment feed gives an idea of which threads are active without relying on the blackbox thread ranking algorithm Reddit provides.