Macbeth wants to have Banquo and Banquo's son, Fleance, killed because of the prophecy the Weird Sisters delivered to Banquo. On the day they told Macbeth that he would become the Thane of Cawdor and then King of Scotland, they also told Banquo that he would "get kings": producing kings in his line of descent, though he would never be king himself. Macbeth wants to prevent the passing of the throne—the position he has worked so hard to acquire—to Banquo's heirs; he is bitter about the fact that he spoiled his soul by killing Duncan only to benefit Banquo's family. He says,
For Banquo's issue have I filed my mind;
For them the gracious Duncan have I murdered,
Put rancors in the vessel of my peace
Only for them, and mine eternal jewel
Given to the common enemy of man
To make them kings, the seeds of Banquo kings. (act 3, scene 1, lines 70–75)
Because he is so bitter about this prophecy, Macbeth decides to arrange for the father and son's murders, but the crime must take place far from his home this time so as not to draw suspicion onto himself. If Duncan is murdered at Macbeth's house and then Banquo and Fleance end up murdered at Macbeth's house, that will look pretty sketchy. To that end, Macbeth tries to get information about where Banquo travels and when so that he can make arrangements to have him killed far away. Then, by confirming that Banquo intends to be back before the banquet that night, Macbeth makes it seem as though he is expecting to see Banquo later on; this, too, seems designed to make Macbeth look innocent.