In the world of the supernatural, the witches in Macbeth represent evil, the reader knows this because they are ugly, deformed, unrecognizable forms, their existence is reflected in their horrible appearance.

"Banquo links them with the devil, calling then the "instruments of darkness," (I. iii. 124) while Macbeth alludes to "supernatural soliciting." (I. 111. 130) In his case, the appearance of the witches has lent his imagination — which had been considering murder, to gain the positions the witches endowed him with — a greater degree of reality."

They are linked with the destructive forces of nature, having the ability to influence nature. They appear and disappear in a flash of thunder.

The witches are seen around a black cauldron, everything about their actions suggests that they are sinister.  Hecate, when she finds out that the other three have been communicating with Macbeth in an attempt to get him to commit evil acts, she becomes angry that she was not included in the process.  She wants to manipulate Macbeth so that he will create more havoc, chaos and destruction.

When Lady Macbeth finds out that the witches have given Macbeth a prophecy regarding his future role, King of Scotland, she immediately encourages him to commit murder. When he appears to be faltering on his decision to kill King Duncan, Lady Macbeth goes outdoors and raises her hands calling on the forces of darkness to inspire her to convince her husband to commit murder.  

She has been included in the count of witches in the play by some critics, so as a representative of the witches, Lady Macbeth seeks help from evil or the devil, in order to fulfill the prophecy that Macbeth received from the three weird sisters.