An interesting question. Banquo's ghost is important for at least three reasons.
First, ghosts appear when someone is dead (obvious, right), and, especially, when someone died with something left undone or been killed in an evil manner. This ghost is an accusation.
Second, it seems like only Macbeth can see this ghost. This isolates him. Is he going mad? Living in a private reality?
Third, when the ghost enters, it takes Macbeth's place at the table. We can take this as a sign—a kind of omen. There's a dead guy in your place; you will be dying soon.