I dont completely understand either. I find it to make sense to revive an old topic to discuss something as opposed something creating a new one, but that rule was here long before I was.
Generally certain kinds topic reviving is OK, but thats often up to moderator discretion. Usually its stuff like "introduce yourself" kinds of threads that dont get outdated regardless of how buried they get. But if someone revives a topic thats been buried for 3 months just to agree with something the last poster said, its quite pointless.
-VGsatomi