Banquo is an important character in the play, and therefore Shakespeare gave the part to an important actor. Shakespeare probably disliked killing this character off so early in the play. While he was writing the script he probably thought of bringing Banquo back as a ghost, thereby getting more exposure for the actor playing the part. Once Shakespeare had established that Banquo was a ghost, he could use the same actor yet again in the scene in which Macbeth confronts the three witches and demands to know more about the future.
I conjure you, by that which you profess,
Howe'er you come to know it, answer me:
Though you untie the winds and let them fight
Against the churches, though the yeasty waves
Confound and swallow navigation up,
Though bladed corn be lodged and trees blown down,
Though castles topple on their warders’ heads,
Though palaces and pyramids do slope
Their heads to their foundations, though the treasure
Of nature's germens tumble all together
Even till destruction sicken, answer me
To what I ask you. (Act 4, Scene 1)
The witches conjure three apparitions and finally show Macbeth eight kings with the ghost of Banquo standing behind them to symbolize that they are his own descendants. Macbeth realizes that his assassination of Duncan was futile, since he only made it possible for Banquo's progeny to inherit the throne.
Shakespeare got extra exposure out of actors in other plays besides Macbeth. In Hamlet, for example, he has the ghost of Hamlet's father appear several times. First the ghost appears to Horatio, Marcellus and Bernardo in Act 1, Scene 1. Then it appears again in Act 1, Scenes 4 and 5. And much later in the play it will appear to Hamlet when he is having his violent confrontation with his mother in Act 3, Scene 4. There was no great necessity for using the ghost again in Act 3, but it gives the actor and an interesting character more exposure.
Shakespeare must have used minor actors many times in different roles in the same play. It seems likely, for instance, that the same boy actor played Portia and Calpurnia in Julius Caesar wearing different gowns and different wigs.