There is a great argument about the significance and relevancy of the witches in the play. Some critics consider them relevant to the plot according to the Elizabethan audience. On the contrary, other critics consider them irrelevant according to the modern approach of the twentieth century. Above all, it is believed that their prophecies are a part of Macbeth's inner conflict even if he is responsible for his actions.
Some critics say that they are not relevant to the play because they have no important role. The play leaves the witches' true identity unclear. Aside from the fact that they are servants of Hecate, they just clearly take a perverse delight in using their knowledge of the future to toy with and destroy human beings. Thus, the witches seems to be just externalization of Macbeth's thoughts and his inner struggle. That is, their prophecies tell Macbeth that he will be King of Scotland, but without saying be a murder. Although they tell Banquo that he will be the father of a line of Kings, he does not care because he is less inclined than Macbeth to ignoble ambitions. It is Macbeth who make decisions to kill King Duncan and Banquo to secure the throne. Also, the witches warn Macbeth from Macduff, but he, instead kills Lady Macduff and her children without any motivation.
The audience is left to think that the witches are simply agents of fate, whose prophecies are only reports of the inevitable. In other words, their prophecies are just remarkably accurate reading of the future. It is hard to see Birnam Wood moving as being a self-fulfilling prophecy in any way. The soldier bears tree's branches while they do not hear the prophecy. Above all, critics claim that Shakespeare use them only to foreshadows what will happen in the play, which may reduce entertainment.
On the other hand, other critics say that the witches are relevant to the play because they create its entire atmosphere. First, they introduce Scotland as setting during the civil war. Also, the witches have a theatrical importance because they are spectacular and means of entertainment. Whenever they appear, the stage directions deliberately link them to unease chaos in the natural world by insisting on "Thunder" or "Thunder and lightening". Such directions set sinister mode for the whole play and present them as the most important supernatural element in the play.
Moreover, the witches are considered the instigators of the play with their words. For instance, Shakespeare has them speak in language of contradiction, which reflects the theme of appearance versus reality. Their famous line "Fair is foul, and foul is fair" is a prominent example. Such speech adds the play's sense of confusion by implying that nothing is quite what it seems. Also, the witches are thought to prompt Macbeth to action through their prophecies. That is, some of their prophecies may be seen as self-fulfilling. For instance, some critics say that he would kill King Duncan and Banquo without their prophecies. Furthermore, they push Macbeth into believing that he is invisible, which leads to his downfall. Thus, he is morally defeated when Birnam Wood moves, and he knows that Macduff is "none of woman born" as their prophecies declare.
In conclusion, the witches may be seen either as real characters who set Macbeth's plot into motion or as just a picture of Macbeth's hallucination. These two sides decide whether they are significance for the plot or not. That is, depending on such debate, the theatrical element may include the acting of these characters or neglect them completely.