They are mysterious characters and Shakespeare, "keeps the witches well outside the limits of human comprehension." They lurk like darker thoughts but also inspire character development throughout the main characters of the play; for example they brought out the real greed and ambition driven attitude of Macbeth.
The witches are all about chaos and causing conflict; whether it be causing an internal conflict for a character or creating conflict between characters. (Like the relationship between Banquo and Macbeth)
The most famous and iconic way to represent evil are monsters and darkness. In the play the three witches embody this sort of representation through their spells and what seem to be magical powers. We can differentiate between what characters seem good and evil in the play by the use of light imagery and how they speak. And the witches are clearly set apart from the rest of the characters from this standpoint.