Why would Shakespeare create such a short scene? What does it do to build tension?

Scene vi sets the stage for Macbeth's greatest fear, and his defeat is insured. Macbeth takes the witches' prophesies at face value, never realizing that things are seldom what they seem—an ironic flaw, given his own treachery. He thus foolishly fortifies his castle with the few men who remain, banking on the fact that the events that the apparitions foretold could not come true. But in fact the English army does brings Birnam Wood to Dunsinane. And Macduff, who has indeed been "untimely ripped" from his mother's womb, advances to kill Macbeth.