Final answer:
Shakespeare includes the story of the witches killing a sailor because of his wife in order to establish themes of supernatural and dark forces and to foreshadow the tragic consequences in the play.
Step-by-step explanation:
Shakespeare spends a significant amount of time telling the story about the witches killing a sailor because of his wife in order to establish themes of supernatural and dark forces in his play. This event serves as an introduction to the witches' power and their ability to manipulate events and people. It also foreshadows the tragic consequences and downfall that will occur as a result of Macbeth's interaction with the witches. By including this subplot, Shakespeare deepens the sense of mystery and intrigue in the play, creating a more engaging and complex storyline.