In the appearance of Banquo, Macbeth's guilt once again manifests itself. One inner conflict impacting Macbeth is the fact that the guilt over the murders of both Duncan and Banquo cannot be escaped. Just as Macbeth had hallucinated about the dagger before the murder, we as readers can see the inner workings of Macbeth's mind after these horrendous acts. It is his own thoughts which haunt him. His imagination will not let him escape from acknowledging (even if it is at an abnormal psychological level) the wrong he has committed.
Still he plans to continue with his evil deeds. This, too, leaves him with feelings of despair and guilt. Despite the fact that Macbeth does not plan to kill Fleance himself, he has set the wheels in motion. This is, yet another, reason for Banquo's appearance. Simply because the witches told Banquo "Thou shalt get kings, though thou be none", Macbeth feels the need to kill Fleance (Banquo's son). Ultimately, he is unable to deal with this decision and Banquo's ghost serves as a reminder for Macbeth's guilt.
Banquo's ghost appears at the table during the feast on the very same night that Macbeth had arranged for both Banquo and Fleance's murder. Although Banquo's ghost does little more than sit in Macbeth's chair, Macbeth's extreme reaction to his presence reveals that his extreme conflict concerning his decision to murder Banquo.
Macbeth's guilt overwhelms and convicts him. He had hired the murderers to separate himself from the actual crime, so perhaps he could make less of his own involvement in the men's deaths.
Thou canst not say I did it: never shake
Thy gory locks at me. (III.iv.62-63)
Banquo's ghost is a not so subtle reminder to Macbeth's conscience that he will never be able to absolve his guilt for the crimes he has committed.