Answer:
The reason why Shakespeare brings Banquo and Macbeth together just before Macbeth decides to kill Duncan is because this interaction could give Macbeth a chance to reconsider his decision to kill King Duncan. Banquo is Macbeth's friend and he is the only person that was present during the Witches' prophecy, so Shakespeare is offering Macbeth a way out of the plan that his wife plotted. Banquo is a very positive character whereas Lady Macbeth is te oppossite. In fact, Banquo warns Macbeth about the witches' words, he tells him to trust them but not to do something to "force" his destiny, but to let life take its course.
"But 's strange. And oftentimes to win us to our harm the instruments of darkness tell us truths win us with honest trifles, to betray 's in deepest consequence" Act I Scene 3 Line 125