Thescenes in the last act shift from Lady Macbeth at Macbeth’s castle to the march of Malcolm and the English forces to Macbeth in Dunsinane to Malcolm and Macduff’s forces entering Birnam Wood to the attack on Dunsinane. The reason for this is simply that you have many significant events occurring in different places within a short period of time.
If I had to pick a dramatic purpose, I would say this shows how all the events leading up to the last act have caused all these events. And these events in Act V, while separate in place in time are all bound to converge. This creates a determined synergy and as the tension increases, Malcolm and Macbeth come closer to battle. The convergence of the scenes in Act V mirrors the rising tension of the play. So, the form that the play takes with these separate but converging scenes mirrors the convergence of consequences.
You might also argue that this convergence is affected by the supernatural aspect. Macbeth may only have been influenced by the witches, but his actions have irrevocably led to this ending: as if it was fated or inevitably caused by Macbeth's desire for power.
This is part of the ambiguity of the supernatural role in the play. How much is determined by fate and how much is determined by Macbeth’s own doing? In other words, all of these scenes are separate but seem to be guided by some outside force, eventually to converge with Macbeth's ending. What is guiding these actions? Is it the fate of the supernatural influences or is it just the result of Macbeth's crimes?