In Macbeth, Act III, scene iii, we learn that Fleance has escaped murder. Why is this insignificant event dramatized?
In Act III, scene ii, Banquo's death is planned. In the next scene we find out that Fleance escapes.
In order to understand the importance of Fleance in Shakespeare's Macbeth, it is helpful to understand for whom Shakespeare wrote this play.
At the time this play was produced, James VI of Scotland had been crowned James I of England, upon Elizabeth I's death (as she had no children).
Shakespeare wrote this play in honor of James I. He referred to James I's genealogy first. And the story of Macbeth was already well-known to the King. (Remember, too, that Shakespeare's gift was not creating the plots of his plays—because they were often based on stories that already existed; it was the way he wrote them that makes them still so outstanding.)
Shakespeare chose to write about James I's ancestor, Banquo; Banquo's "issue" continues through his son Fleance, who escapes Macbeth's plot to destroy Banquo's line.
If you remember the witches' first set of predictions, they speak to Macbeth and Banquo. Whereas they tell Macbeth he will be king, they tell Banquo that while he won't be a king, he will father a line of kings. Macbeth believes what the witches told Banquo—and this is why Macbeth wants Fleance killed. He does not want Fleance to grow up and return one day to wrest the throne from him.
This prediction is revisited during the witches' second meeting with Macbeth: they share a vision of Banquo's sons (down through the ages). It is at this point that Macbeth truly understands that he has sacrificed his soul (in killing Duncan) for nothing. His family's rule will end with his own death: Macbeth has no children. But Banquo's son's sons will inherit the throne of Scotland.
While Fleance has a seemingly unimportant role, it is through his character that Shakespeare can connect Banquo and James I together. In this way, Shakespeare compliments the King—Banquo was a nobel warrior—a great man of conscience and integrity.
This event is not insignificant. Macbeth is with Banquo in Act I when the Weird Sisters prophesy that Banquo "shalt get kings, though [he] be none." So, now that Macbeth has murdered Duncan to gain the fulfillment of his prophesy, he is definitely worried that Fleance could usurp him or his sons (when and if he has any).
In Act III, scene i, before Macbeth instructs the murders who will go after Banquo and Fleance, he has an important conversation with the audience (a soliloquy). Here he confides that he is King, but he is not "safely" King. Of the witches prophesy he says:
They hail'd [Banquo] father to a line of kings.
Upon my head they placed a fruitless crown,
. . . No son of mine succeeding. If it be so,
For Banquo's issue have I filed my mind;
For them gracious Duncan have I murdered. . .
And so, Macbeth shares with the audience that, unless he murders Banquo (so that he cannot create any more sons) and Fleance (his one living son), the costly actions he has taken to make himself king will be for naught except to crown Banquo's line. Since these murders are so key to Macbeth's preventing Banquo's prophesy from coming true, the scene in which the murders are supposed happen is hardly insignificant.
This scene is also important to the dramatic suspense of the play. Shakespeare has in keeping with dramatic tradition had the murder of Duncan happen offstage, so this is their chance for some onstage gore, as the Murderers kill Banquo in full view of the audience.
Another interesting element of the scene is the appearance of the Third Murderer. Shakespeare makes sure to call attention to this curious character, because the First Murderer asks him, "Who did bid thee join us?" The Third Murderer says, "Macbeth." Is it Macbeth himself? One of the Weird Sisters? Lady Macbeth? Or simply another Murderer sent to make sure the job is done? This is a question that must be decided by each theatre company producing the play.
The escape of Fleance is definitely not a minor point, and this point is made more clear when one of the Murderers reports to Macbeth in scene iv that Fleance has escaped, and Macbeth says:
Then comes my fit again. I had else been perfect. . .
But now I am cabin'd, cribb'd, confined, bound in
To saucy doubts and fears.
The escape of Fleance leads his mind to a "fit" which could well account for the vision of Banquo's ghost that he is about to see. This scene is the turning point in the play. After this, Macbeth will be exposed as a murderer and deserted by all his comrades, left to fight for his ambitions alone.