On my computer it jumps to the last post I looked at. That helps me establish context.
Didn't notice that explicitly, as it's on the page at the first post I haven't seen. Unless the next "new" post is the first on the next page, then it goes to the last on the previous page.
Also, on many pages that list topics, there is "=>" link near the topic's title. That link would lead you to the last post you've read in that topic. You can use it to quickly pick up where you left off and read just the new replies, for any topic.