The point is not to force people to show up an hour early, but to allow those who want that to go inside. That makes it easier for many of us who would rather show up a few minutes before 8 and not find a stress-inducing line. You must have noticed that lines begin an hour or more before curtain time. I don't understand why anybody - tourists, presumably - think that's necessary, but for some it seems to be part of the event.
