Basically on a certain (configurable) day of the year all topics posted before a certain (configurable) day get marked as Archived. This happens once a year.
Example on the 120th day of the year all posts from before the 30th day of the year will get archived.
In our case we are a college and conversations that span over a year gather kruft of no longer valid information. It is better to begin again the fight over smoking on campus again in the context of current policies. The old conversation still exists for reference.
Another valid example is, a game company has a beta or alpha for a game. Three years later, the game is no longer like the beta or alpha and much different, if anyone even plays it any more? So why keep the alpha and beta categories at all…