Macbeth needs to know, with some amount of certainty, the approximate time Banquo and Fleance will be returning to Macbeth's castle. Remember, Banquo and Fleance come under a surprise attack, so it is important (to Macbeth) that he arrange for the murderers to be in the right place at the right time. Most importantly, however, this sets up the transformation of Macbeth from a superstitious and somewhat impulsive killer (Duncan), to a calculating murderer. The murder of Duncan is much different than the murder of Banquo and Fleance. Macbeth slips further and further into the darkness as he plots the death of his closest friend, Banquo.
Macbeth questions Banquo about his plans for the day and evening to make sure Banquo will show up at the feast that night, so he (Macbeth) can have Banquo killed.