That way there's no excuse of "I have to stay on because $BIGPROJECTX needs my help." There's literally nothing going on. Go home. Get a life.
Such drastic measures are necessary, unfortunately, because just a few folks working 1.5x hours can end up sucking everybody else in. If you make it mandatory and set a designated date, it's much harder to have the game-playing.
Note that this is also a good way for people who work together to do things together. After all, you're all off during the same times :)