How important is Banquo's role in Macbeth?

Although Banquo is not one of Macbeth's main characters, he does play a very important part. Banquo represents goodness and purity; he is a loyal friend to Macbeth, and in turn is trusted by his best friend. Unfortunately, His discovery of Macbeth and the witches changes the course of their friendship. Suddenly he is no longer fa friend, and because of the witches' prophesies becomes a threat to all that Macbeth desires to attain. When Macbeth hears that it is Banquo's descendants who will be kings, he becomes suspicious and withholds his friendship. Macbeth also realizes that because of what banquo has witnessed, he has good reason to suspect him of murder.