The movie shows how close to defeat they were and what would be the cost of defeat. When the king rode out to the enemy, it was a last ditch attack where he decided he would die attacking rather than die trying to bar a door shut. The futulity of that attack was not brought out in the book.
Because the movie correctly describes the battle, the moment when Gandolph arrives (the one who rose from the dead bringing light to the world) to turn the tide actually makes more sense.
I don't get it. How can a departure from the book be more true to the story? The book is the story. What am I missing here?
(Don't get me wrong, I love the move, enough to have paid to see it six times so far.)