The Witches in Macbeth; Why are they in it and what do they add to our understanding of the play?

Morality
The witches are key figures when it comes to Morality in Macbeth. Firstly they appear as completely immoral through there murderous actions ( I, III, 18-20) and their outer worldly superstitious actions (building the cauldron). This provides a useful example of immoral figures which produces a direct contrast to that of Duncan, highlighting his grace. It also highlights and explains Macbeth's downfall to immorality, as it gives the audience something to compare him to.
Their actions also question the moral actions of Macbeth and humans themselves. This is because the way in which the witches gain power is so similar to Macbeth. When the 1st witch is annoyed by a sailor calling her 'a witch' and 'mounch'ing she kills her husband 'I'll drain him dry as hay:/ sleep shall neither night nor day/ hang upon his penthouse lid' (act I, III 18-20). This proves that in order for the witches to gain power, which is the fear that others have of them, they kill people. This is similar to Macbeth who feels that in order to gain power he must kill the king, along with with many others.

Structural and Propleptic Devices
Including Act III scene V, the witches are in four scenes. The first acts as an introduction, both  to their characters and to the play as it is the first scene. They are the first to mention Macbeth which tells the audience of their omniscient qualities. They also introduce the idea of nature being turned on its head through 'fair is fouls, and foul is fair'. The second scene they are in, Act I scene III, is where they prophesies to Macbeth and Banquo, and some would argue that without this scene Macbeth would have never thought about killing the king. The next scene with Hecate, act III scene V, is arguably not written by Shakespeare, therefore cannot always be considered when trying to answer the question of the witches impact of the play. Their final scene, Act IV scene I, is the scene with the apparitions and the famous cauldron making. All of the scenes that the witches are in act as proleptic episodes, which foretell the play. This device gives the witches some of their power.
This device is gives them a similar role to that of the chorus in Greek plays. The chorus acted as the neutral figures, who were often had powers, like 'the clouds' by Aristophanes who were a deity. However the witches represent almost a 'backwards chorus' because rather than summing up what has happened in they play, which is often their role, they foretell it. There supernatural power could also be used to liken the to the fates.

Supernatural influence
Macbeth is often cast as a 'gothic' play, and this is partly because of the supernatural in the play. The witches are Shakespeare's main link to supernatural in the play. They seem to possess foresight as they predict Macbeth's reign and thane-ship 'hail to thee, Thane of Cawdor!/... that shalt be king hereafter' (I, III, 49-50). Also the basic fact that they are described as witches associates them with evil and the devil. This belief was even stronger at the time Shakespeare wrote the play because many people, including the king believed that they were real.
I could be argued that the witches in Macbeth don't actually posses any real power, rather that Macbeth gives them their power. This is because if he did not take their words so readily for truth and killed the king, there is a chance that he would not have become king. In this way it is his actions, not those of the witches that give them their supernatural power.

The question of fate
When assessing the witches influence in the play it is key to imagine the play without them at all. Without them Macbeth would never have heard their prophecy claiming he would be king, which brings forth the question would he still kill Duncan? If you believe that he wouldn't then you would have to conclude that it is the witches fault that Duncan dies and hold them responsible for the downfall of Macbeth.
If you conclude that Macbeth would have done it anyway because of his crazy ambition, we have to conclude that the witches act only as a prompt to Macbeth. This reduces their power and influence in the whole play greatly because it suggest that the play could be complete without them.
However this is debatable as many people would argue the first and many the second point.