I've found it really handy for threads that have images in them. That way once you see it, it contracts the post and you don't have to have the image load everytime you revisit the thread.
The only drawback it seems to have is if someone edits a previous post. With it being collapsed, you'll never know.
The only drawback it seems to have is if someone edits a previous post. With it being collapsed, you'll never know.