Shakespeare uses this scene to portray how brave Macbeth and Banquo are as well as their impression King Duncan. It is also included to give the reader an original idea of how Macbeth is like and what he will become throughout the rest of the play.
At the castle, the captain tells king Duncan about Macbeth's bravery at war.
Dismayed not this our captains, Macbeth and Banquo? (1.2.34)
Yes, as sparrows eagles, or the hare the lion (1.2.35)
Doubtful it stood, for brave Macbeth—well he deserves that name— disdaining fortune, with his brandished steel, till he unseamed him from the nave to th' chops (1.2.7)
The witches are characterized as ruthless and short tempered which is seen by their actions and words. Although they are violent with others, they are accepting of each other which is seen through the witches desire to help another witch with creating mischief.
Meanwhile, as the witches are waiting for Macbeth:
“Give me" quoth I. “Aroint thee, witch!” the rump-fed runnion cries. Her husband’s to Aleppo gone; but in a sieve I’ll thither sail, I’ll do (1.3.4)
Where hast thou been, sister? (1.3.1)
Killing swine (1.3.2)
I’ll give thee a wind (1.3.11)
Sister, where thou? (1.3.3)
This line by Banquo tells the audience how straightforward Banquo is as he says such a line that could be taken in an offensive way to people he has never met before. It also gives insight on the witches' appearance; they have beards, wear ragged clothing and look as if they are not from this planet.
The witches meet Macbeth and Banquo who are returning from battle.
What are these? So withered and so wild in their attire, that look not like th' inhabitants o' th' Earth. You should be women, and yet your beards forbid me to interpret that you are so (1.3.39
Macbeth and Banquo meet king Duncan in the castle.
This shows the amount of gratitude the king has for Macbeth. It also gives the audience an update on Macbeth's thoughts on the witches' predictions so far. Thus far, he does not have much of an ill will towards king Duncan.
O worthiest cousin, only I have left to say, more is thy due than more than all can pay (1.4.15)
Our duties are to your throne and state children and servants, which do but what they should, by doing everything safe toward your love and honor (1.4.24)
Lady Macbeth reads Macbeth's letter:
This shows how Lady Macbeth does not believe that Macbeth is able to become king. She thinks that Macbeth’s nature is to kind and weak to seize the opportunity. The opportunistic side of her is waiting for Macbeth to return home so she can persuade and manipulate him to kill king Duncan and become king himself.
Yet do I fear thy nature; it is too full o' th' milk of human kindness. Hie thee hither, that I may pour my spirits in thine ear and chastise with the valor of my tongue (1.5.1)
Shakespeare created this intense soliloquy to show Lady Macbeth's masculine nature. She wants to become a male just to kill king Duncan which shows her ambition. The opportunistic character will let nothing get away that will help her fulfill her goal.
Come, you spirits that tend on mortal thoughts, unsex me here, and fill me from the crown to the toe top-full of direst cruelty. Make thick my blood. Stop up the access and passage to remorse, that no compunctious visitings of nature (1.5.30)