Although Shakespeare has Duncan murdered offstage and Macduff's family also murdered offstage, he enacts the murder of Banquo onstage because he wants the audience to see with their own eyes that Banquo is really and truly dead. This will assure the audience that it is Banquo's ghost who appears at the banquet scene in Act 3, Scene 4. Otherwise some spectators could get the erroneous idea that Banquo, albeit somewhat tattered and bloody from the assault, has somehow managed to survive and put in an appearance. Shakespeare also has one of the Murderers appear at the door to the banquet hall to verify that Banquo is unquestionably dead. The actual murderous assault convinces the audience that Banquo is dead, but the Murderer must convince Macbeth, who only knows about it from hearsay.
Notice that the three murderers apparently all attack Banquo. They do not consider Fleance a problem. But this gives Fleance, probably played by a boy about twelve years old, a chance to run for his life, which is what his father encourages him to do. Once the boy starts to run, the murderers have no chance of catching him because they are too involved with Banquo, and also because they probably couldn’t catch a young boy who was running for his life.