There are two parts to the witches' chants. Firstly, they provide the prophecies that set in motion Macbeth's journey. When they predict that he will be Thane of Glamis, Thane of Cawdor and finally King, he doesn't believe them. However, when the first two predictions come true, this inspires him to help fate out and kill Duncan in order to fulfill the final prediction. At this point the witches also predict that Banquo's sons will be kings. This foreshadows Banquo's death at the hands of his best friend. By the time Macbeth is ordering the murder of Banquo, his character has completely changed from war hero to murder and it is the witches' prophesy that is the catalyst for this change. The witches also provide the three warnings to Macbeth. In showing him the three apparitions, they reason that they have done everything they can to save him from his demise. Macbeth's reaction to their warnings foreshadows his downfall.
The witches also make a number of insightful observations about human nature. The most significant of these is "fair is foul and foul is fair." This suggests that nothing and no one is entirely good or entirely evil, that everything has some proportion of both and that sometimes it can be difficult to distinguish between good and evil. '
Shakespeare wrote in a specific context in terms of society's perception of witches. Therefore, his characterisation of the witches, including their dialogue, is a valuable insight into the stereotypes, culture and values of Elizabethan England. Think about things like the symbolism of 3. Choosing to have 3 witches is suggesting that they are the antithesis to the Trinity of the Catholic church. The witches also discuss their familiars (toads, cats, other animals). This is a clear reflection of a prejudiced idea held during the Elizabethan period - that an old woman with a pet was communicating with the devil via the pet.