Why did Shakespeare spend so much time telling us a story about how the witches killed a sailor because of his wife? (Macbeth)

The witch, with the help of her sister witches, will control the winds so that the sailor won't be able to come into port. Contending with the storm, he won't sleep and so he will "dwindle." The witch gloats that "Though his bark cannot be lost, / Yet it shall be tempest-toss'd" (1.3.

oh i didn't know thx for tell me hallelujah