Why do you think the witches tell Banquo his sons will be king?

The way this question is phrased makes it seems like you are being asked to give an opinion on why the witches would tell Banquo he will be the father of kings. The play does not explicitly answer this question. In act 1, scene 3, Macbeth and Banquo are walking across the heath after winning a battle against Norway. Three witches appear out of nowhere and hail Macbeth as "Thane of Glamis," "Thane of Cawdor," and "king hereafter." The entire prophecy given to Banquo by the witches in this same scene reads as follows:

First Witch: Lesser than Macbeth, and greater.
Second Witch: Not so happy, yet much happier.
Third Witch: Thou shalt get kings, though thou be none:

The prophecy reads like a riddle. How can someone be both "lesser" and "greater" than another person? To break down the witches' predictions, we have to actually start with the final one. That Banquo will "get kings" but not "be" one means that he will be the father of kings or ancestor of kings (he will "beget" them), but he will never be king himself. This means that he will be "lesser" and "Not so happy" as Macbeth because Banquo will never be the crowned ruler of Scotland. However, he will, at the same time, be "greater" and "much happier" than his friend. This is because his legacy will live on and he will not be corrupted by power like Macbeth will be.
The question is whether the witches know ahead of time exactly what will happen after telling Macbeth he will be king (this includes trying to kill Banquo and having Banquo's son Fleance killed). Did they set the entire tragedy into motion or did they just know what Macbeth would do with the information, then watch it play out? How you answer this question has much to do with how you would answer the question of why the witches even tell Banquo about his descendants. The information certainly eats away at Macbeth, both leading to the aforementioned murder of Fleance and attempted murder of Banquo in addition to a tantrum once Macbeth sees a vision of the dead Banquo's face multiplied many times in a mirror in act 4. The information adds to Macbeth's paranoia and his feeling that his reign is not that significant because he will not have any heirs. We can either believe that the witches take some sort of sick pleasure in seeing Macbeth's downfall (in which case they would know and enjoy his reaction to the prediction about Banquo) or that they simply provide information and Macbeth is solely to blame for the actions he takes as a result of that information.