As a director of the play, one of the fascinating choice to be made is exactly how to present the weird sisters aka witches. Their physical presentation depends on when the director chooses set the play.
In Shakespeare's day, Macbeth would have been presented in modern dress or Elizabethan. Visually this would have presented a very understandable world since as stated above, witches were real women to these people. His audiences would have also been familiar with the Anglo Saxon word from which weird derives. It meant fate. Shakespeare has blended two things here, current Christian beliefs with more archaic ones.
This marriage opens up the interpretation of these women who could be anything from oracles, like the Oracle at Delphi. They can predict but not control events. They could be real witches who practice the dark arts and call upon Satan through their familiars. They could also be mentally unstable women, outcasts, who wander around picking up fragments of information, who believe they are witches since that is what society has branded them.
The beauty of Shakespeare is that there is no one correct answer to the questions his work prompts but many. He really was a man for all time.
It is also worth considering whether the witches aren't just spirits or figments of the imagination. Shakespeare goes to great lengths when they encounter Macbeth, both in Act I, scene iii and in Act IV, scene i, to have Macbeth comment on the fact that they have disappeared before his very eyes. It is true that, in Act I, Banquo also sees them, but there is not another soul in the play who does. This is made very clear in Act IV, when Macbeth questions Lennox as to whether he hasn't seen them passing by him.
Witches, in Shakespeare's day, according to the judgement of soceity, were actual real people, most often women. We know something of the condemning and burning of witches that ran rampant by our history of the witch trials of Salem, Mass., and these same sorts of events were taking place throughout Europe during Shakespeare's day. So, if witches were commonly understood to be ordinary people with access to evil magic, why would Shakespeare create witches in this play which could be considered to be figments of the imagination?
One answer to this question could be that they are meant to represent the evil impulses that are unleashed in Macbeth and Lady Macbeth upon considering the possibility that Macbeth could become king, upon giving in to their greedy ambition. In this case, the witches aren't an external force that lead Macbeth astray, but a manifestation of his own darker nature.
In Macbeth, the three witches are representative of Fate and are the catalyst for change in the play. Macbeth believes the verity of the witches' prophecy after he returns from battle and is given the title Thane of Cawdor by King Duncan. Macbeth then wonders when he will become king. The witches have not told Macbeth the details of these occurrences, so Macbeth only knows that he will become king, not how. The witches do not suggest in any way that Macbeth has to do evil deeds to become king--Macbeth's personal ambition gets the better of him and he allows himself to submit to evil. Later, the witches also present another prophecy to warn Macbeth of the dangers that will be presented to him. Again, they simply tell him the facts without any details on how to interpret their message. Macbeth thinks that the later prophecy cannot come true because he misinterprets the message. So, the witches appear as elements of evil in the play, yet they serve more as catalysts for Macbeth's change in the play.
The inclusion of witches in Macbeth, written for the new king James 1, is for the sole benefit of that king. James was fascinated by witchcraft, and wrote his own treatise on the subject.