'Macbeth', like many of Shakespeare's works, had its basis in historical fact but was adapted to suit his own authorial intent and to allow flexibility of characterisation and theme.
Shakespeare composed the play in the time of James I, and was likely to have been presented to the monarch. It was important that the play looked favorably on James' monarchy, or Shakespeare could have found himself in great difficulty!
James I was descended from Banquo's line, and thus the character of Banquo is a thoughtful and kind man within the play. In the account of Macbeth from Holingshed Chronicles of England, Banquo is involved in the murder of King Duncan. It is a wise political move on Shakespeare's part for these events not to be included in his play.