We need to understand who the witches are and what they represent overall to understand this scene better. Whenever the witches are shown in the play <em>Macbeth</em>, they are usually foreshadowing later events in the play. In the play, when the witches meet with Macbeth, they call him "Thane of Glamis," "Thane of Cawdor," and "king."
This scene is significant overall. Thane of Glamis is Macbeth's original title, so he anticipated being called that. However, he had not, at this point, received word of King Duncan's choice to name him Thane of Cawdor, so he is confused by this proclamation. He is also confused by being called king as they inform him he will one day be king.
It is during this scene that Macbeth is informed of his new title and he realizes that the witches have essentially told him his fortune. The timing of all this is very significant because it proves to Macbeth that what the witches tell him is true. Without this, it is unlikely that he would have believed the "prophecy" that they tell him of his future.
