During the Elizabethan Age, people believed very strongly in the supernatural realm because it was an important part of the Great Chain of Being since whatever affected one realm affected others. Moreover, Elizabethans strongly believed in ghosts and their influence in the lives of humans.
Shakespeare's use of ghosts in Macbeth greatly furthers the character development of Macbeth himself as well as progressing the plot and playing to the beliefs of his audiences. For, Macbeth is a tragedy of the imagination as "Nothing is what is not" for Macbeth and the phantasmagoric realm, a constantly shifting of things seen and unseen, drives both character development and plot. Certainly, too, this supernatural realm of ghosts and apparitions greatly enhances interest in the narrative of the play as the witches predict various changes and influence Macbeth's imagination until he envisions daggers and the ghosts of those he has murdered. Also, the witches' prophesies certainly seduce Macbeth--
If chance will have me King, why,
chance may crown me,
Without my stir. (1.3)---
and lead him into his unnatural and murderous state as he imagines that Banquo's ghost wishes to avenge itself and, later, that Birnam Wood moves toward him for his defeat by Malcolm.
At the time that Shakespeare was writing Macbeth he was fully aware that society's belief in the supernatural would influence how they viewed this production. Londoners in the 16th and 17th centuries strongly believed in the supernatural presence of ghosts, and the power of magic. Therefore, as they viewed the witche's scenes of Macbeth it is likely that they would have thought actual magic was taking place on stage.
The Globe Theatre, where Shakespeare's plays were primarily performed had a trap door installed in the floor allowing the witches to actually appear from underneath the audience, adding to the illusion of magic. Shakespeare may have been making allusions to the witches rising from hell by bringing them up through the floor.
As the witches recite their incantations in Act 4, scene 1:
"Double, double, toil and trouble,
Fire burn, and cauldron bubble."
It is likely that the audience was experiencing a fair amount of terror or trepidation, as they would have believed they were witnessing actual spells taking place, by actual witches. This added to the tension, and mood that Shakespeare was trying to set throughout the play.