The various events in Macbeth drive the plot forward. We learn of the witches at the onset and will become very familiar with their mantra of "fair is foul and foul is fair" (I.i.10) such as it encourages Macbeth's unchecked ambition which "o'er leaps itself."(I.vii.27) Their involvement in the plot helps to foreshadow events and suspense is increased as Macbeth seeks them out when he needs reassurance.
Macbeth is haunted by Banquo's ghost and is convinced he "does murder sleep." (II.ii.36) By making him less reliant on his wife as he seeks to further his gains without her, keeping her "innocent of the knowledge," (III.ii.45) the plot is broadened as Macbeth is no longer Lady Macbeth's puppet although he still needs her to explain his madness to his guests as his "strange infirmity" (III.iv.86) - his hallucinations - almost cause him to unravel. Due to his new found independence, he becomes more reliant on the witches to ensure that his ambition to be king remains intact.
The audience becomes aware that MacDuff and Malcolm will return to fight Macbeth who thinks his resolve has been strengthened by the witches but in fact, the loss of his wife reduces his story to "a tale, told by an idiot,.... signifying nothing."(V.v.26-8)
The entire plot of Macbeth proceeds quite quickly, with little time for contemplation. Shakespeare introduces the scene with the Porter as a means of light relief from this otherwise, tragic portrayal of a man, a hero, ruined by ambition.