Excellent question. Let us remember that when Banquo appears to Macbeth during the banquet scene in Act III scene 4, Macbeth has just personally organised to have Banquo disposed of by using trained murderers that he has obviously persuaded to kill Banquo through manipulation. This represents an escalation of Macbeth's level of evil. Let us remember that when he killed Duncan, it was something that he himself did personally, and it was all organised by his wife. The fact that in Act III Macbeth shows that he is becoming more cold-hearted about murder through employing others to kill for him and coming up with his plans himself represents his descent into evil. It is therefore fitting that, just as Macbeth was haunted by the sight of the blood on his hands in Act II after killing Duncan, Macbeth should be haunted by the presence of Banquo in this scene.
At the same time, we could also point out a deeper significance in the appearance of the ghost of Banquo. Let us remember that Banquo is used as a foil for Macbeth in the play, in particular in the way that the two characters respond to the prophecy. Banquo is shown to be a character who does not act in response to prophecy, whereas Macbeth does act. In particular, the prophecy that the witches gave to Banquo concerning his heirs becoming kings has just been given extra credence by Fleance's escape. As such, Banquo's presence acts as a rebuke to Macbeth, reminding him simultaneously of how Banquo responded to prophecy but at the same time taunting Macbeth with the failure of his attempt to change the future by trying to kill Banquo and his heirs.