There is a reason that this is the final piece. A photo finish can only be accomplished once the other two fundamentals have been completed.
Think about how many times you see a PGA/LPGA Tour players reaction to a golf shot before the camera changes to follow the ball. It doesn't happen often and contrary to what some believe, they aren't just doing it for the cameras. There is a function to the pose.
A golf swing without a pose is like a movie without an ending. Good, bad, or indifferent, there needs to be an ending. The same applies to your golf shot. In order to have a photo finish, you have to have intent on where you want to finish. This is just another way to block out all of the irrelevant things you can't control and keep your mind present and on the aspects of the game that you can control.
Also without you knowing it, holding your finish adds confidence. When you hold your finish, you allow yourself to process without any emotional attachment. You hit a good shot and you get the luxury of standing there and appreciating it. You hit a bad shot and you get to stand there and think about what could have went wrong and what you are going to do differently next time, all before exiting the golf shot. If you can get yourself to hold your finish consistently for the duration of 18 holes, you will be amazed by the amount of confidence you will have in yourself.