What is the role of the witches in Macbeth and the atmosphere they create?
In Macbeth, the witches serve as a catalyst for Macbeth's fall into evil and as a symbol for Fate. At the beginning of the play, the witches tell each other that they will meet again when the battle is over, and a couple scenes later, this does in fact happen. The audience thus knows that the witches are a sign of supernatural happenings and trust that the witches' prophecies will be true. Macbeth also believes the verity of the witches' prophecy when soon after he is given the title Thane of Cawdor as they predicted. At this point, Macbeth starts to think that Fate is on his side, but he is persuaded by his wife and his inherent sense of ambition and greed to do ill deeds to gain the throne. However, the witches do not say that Macbeth has to do ill deeds to get to be king--they are simply the mouths of Fate that tell him what will happen. In the play, the witches create an atmosphere of mystery and evil.
The witches also serve a function or role as regards the audience. They would have been the draw for audience in terms of the magical acts they performed onstage and special effects needed to create the effect of these acts. We aren't sure, for example, how they would have disappeared after their first meeting with Macbeth and Banquo (these two characters discuss the witches disappearance into thin air), but it probably involved a trap door.
What with the magical potion that concoct onstage, the calling for Hecate to appear, the parade of spirits that present to Macbeth -- they certainly would have provided as much entertainment for an audience in the early 1600's as any special effects we marvel at in a film like Avatar today. So, they served the very important role, also, of creating an atmosphere of magic by amazing and entertaining the audience with special effects.
Putting the play into perspective, Shakespeare's audiences believed in the supernatural and witchcraft. The king himself was interested in the subject. For playwright, William Shakespeare, it was a natural structural element.
Who are these women? They are called the weird sisters, weird coming from an Anglo Saxon word which meant fate. In this sense, they do not "make" things happen, they just function as a kind of oracle. They tell Macbeth that he will be king in the future. They do not tell him to kill Duncan.
When Macbeth attempts to kill both Banquo and his son, Fleance, he has interfered with fate and therefore must pay the price. He is led to believe that he is invincible but he is not.
Exactly who these women are is up to interpretation. Their function is to create a supernatural atmosphere where "nothing is but what is not".