William Shakespeare wrote Macbeth in the 17th century. The king at the time was King James and he believed witches had very special powers, they could make people ill or even kill them, they were also believed to make the weather change and make the nights longer with their curses of evil. So obviously if they thought someone was a witch they would test them to see if their prediction was true. But more often than not if people thought you were a witch you were more or less already dead because they used to tie the suspect in the middle of a rope and then pull them into the middle of a river or pond and if they untied themselves and flew away they were a witch and they"d kill them, and if they didn't they weren't they"d stay in the middle of the pond or river and drown, so they"d die aswell. So if you were believed to be a witch were virtually dead anyway.
Shakespeare worked on the element of the supernatural to entice his audience into watching his play right to the end. This worked well because the Elizabethans were afraid of witches because of their abilities and what they could do if they were angry for any reason. Shakespeare used James treaty on witchcraft to develop his ideas and to count favour with the audience.
The opening scene of the play sets the mood and other aspects for the rest of the play. The witches or "unnatural hags" use language and elements that are linked with witchcraft and the supernatural and go against God and his natural order, here the audience realise that there is only evil present and that no good will prevail above evil. This is where Macbeth first appears in the play with Banquo returning from a victorious battle against rebels, it was at this time they encountered the three weird sisters upon an abandoned heath and they give Macbeth two predictions and Banquo one, but Banquo had to ask for his prediction. This shows that the witches were only interested in Macbeth because he must have a side of evil and deceit and therefore the witches could play with his mind.