The scene has a number of functions. First, it introduces a break from Macbeth and his woes. He has just seen Banquo's ghost and was terribly frightened, so much so that his wife had to admonish him for being "unmanned in folly" and for breaking up the banquet with his disorderly conduct.
Second, it provides a link between the first part of the play (in which Macbeth met with the witches) and what we shall soon find are the true consequences of that initial meeting. Macbeth will soon learn what it is to indulge the powers of evil. He has already had a taste thereof in his encounter with Banquo's ghost and has already acted on the ambiguous promises and predictions the evil sisters have made.
The introduction of Hecate, the goddess of witchcraft and magic, adds to the drama and is portentous. Her presence is an indication of worse things to come; Macbeth's already ruthless and bloody acts assume a new level of atrocity after her address to the weird sisters. He orders the assassination of Macduff's entire family and his servants.
It is clear from Hecate's anger that she saw Macbeth as a juicy fruit, ripe for picking, and that her minions acted without consulting her, thus denying her the opportunity of corrupting him herself.
Hecate makes it clear that what has gone before was mere trifling. She states that
Great business must be wrought ere noon.
Her statement foreshadows Macbeth's later meeting with them. On this occasion he will be inspired to take greater risks, for he will be led to believe that he is invincible and unconquerable. Hecate also states that the meeting will inform Macbeth of his destiny. He, as we know, will be completely misled by the predictions of the apparitions that the witches will summon. His doom is surely sealed. Hecate maliciously, and probably with relish, states:
...shall raise such artificial sprites
As by the strength of their illusion
Shall draw him on to his confusion:
He shall spurn fate, scorn death, and bear
He hopes 'bove wisdom, grace and fear:
And you all know, security
Is mortals' chiefest enemy.
It is clear that the forces of evil have found in Macbeth the perfect vehicle through which to exercise their malice. This scene accentuates the depth the witches' perfidy and their utter confidence that Macbeth will allow himself to be misled and used to fulfill their purpose.
In Act 3, Scene 5, Hecate scolds the three Weird Sisters for messing with Macbeth. This scene serves to increase the suspense and foreshadow later trouble for Macbeth.
Act 3, Scene 5 mainly serves to increase the suspense and establish a mood of darkness and violence. In this scene, Hecate confronts the Weird Sisters about their interference with Macbeth.
How did you dare
To trade and traffic with Macbeth
In riddles and affairs of death …? (p. 54)
She tells the witches she is going to step in and take care of things now, and they will meet again with Macbeth and he will “come to know his destiny” (p. 54). While the first scene involving the witches was kind of playful, we get the idea that things are serious now.
We know that trouble is coming for Macbeth, because Hecate describes him as a “wayward son” and tells the sisters that she plans to be busy, spending the night involved in “a dismal and a fatal end” (p. 54). This scene serves as foreshadowing of the witches’ involvement and the darkness remaining in the play.