The line links the witches to Macbeth and contrasts him with Banquo.
Shakespeare chooses to have Macbeth enter and repeat the witches’ line in order to tie him to the witches. This foreshadows how the witches will impact Macbeth’s life. We have seen that he is a brave and loyal solider. Now we are about to get a different view of him.
Macbeth’s line ties him to the witches and contrasts him with Banquo. Banquo is immediately suspicious of the withes.
What are these(40)
So wither'd, and so wild in their attire,
That look not like the inhabitants o’ the earth,
And yet are on't? Live you? or are you aught
That man may question? (Act 1, Scene 3, p. 12)
Macbeth, on the other hand, asks them to speak. When the witches make their prophecies, Banquo is concerned but Macbeth is excited. Banquo cautions Macbeth, but he does not listen.
But ’tis strange;
And oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's(135)
In deepest consequence—
Cousins, a word, I pray you. (Act 1, Scene 3, p. 15)
At this time, Macbeth is just thinking about how the witches got one of the prophecies right, and he has been promoted to Thane of Cawdor, so the other one should be right too, and he should be king.
This is the beginning of the end for Macbeth and Banquo, as Macbeth goes the way of the witches and Banquo opposes him and them.