The Why Now of the Movie

There's this famous memo that's out there of David Mamet talking about. Why should it why must a scene happen now and at no other time on the story's timeline? Will it work? And I think you guys agree with me like you can use that for the story itself. Like I get something that's not really expressed in the movie, but if you've seen it a hundred times, you start to realize. That is the ticking clock starting.