In Act IV, scene 1 of "Macbeth" why is the witches' chant given in detail?macbeth act 4

First, remember that Shakespeare wrote "Macbeth" to compliment King James I, who had become the patron of Shakespeare's acting company in 1603. Shakespeare referenced many of the King's ideas about government and religion in the play. One of King James' interests was witchcraft, about which he wrote extensively. Shakespeare took information from two of the King's accounts on witchcraft and incorporated these ideas in the play. For example, in 1.2 the idea that witches sail in sieves or that they can cause storms on sea is taken from the two sources on witchcraft authored by King James. Also, in 4.1 the details of the charm elaborated upon by the witches is taken directly from the King's account of a witch concocting a potion to be used against the King himself.
Secondly, the detail used to describe the spell in 4.1 helps to show the progression of Macbeth's downfall. In the play's first scene, the witches briefly plan a meeting with Macbeth. In 1.3 they meet with Macbeth and pronounce their alluring prophecies. By the end of 3.5 Macbeth determines to seek the witches himself for the truth of his destiny. He is willing to put his soul at risk by putting his faith in the evil witches. So in 4.1, the last time we see the witches, the spell will bring forth the 3 apparitions which beguile Macbeth into his final descent into evil.