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.
There are definitely two reasons for the appearance of Banquo's ghost at the banquet. First, he is a reminder of Macbeth's guilt and foreshadows even more deaths to come as well as Banquo's lineage and claim to the throne. Second, since the guests see Macbeth's reaction, they can interpret it for themselves. They realize that something is rotten in the state of Scotland and can conclude that Macbeth is guilty of murder, perhaps more than once. Ultimately, Banquo's ghost confirms Macbeth's sins, both to himself and to his peers.
One reason of the importance of Banquo's ghost is that it symbolizes the down fall of macbeth coz only him can see and it also takes his place in the table showing that banquo's sons shall be king.
Second is that it shows macbeth's guilt and the quest can see that there is something wrong with macbeth.