What is the importance of the witches before and after the apperance of Banquo and Macbeth?

The play opens with the witches meeting on the heath and saying that they are going to meet Macbeth. Then they tell us at the end of Act 1, sc. 1, that appearances will be deceptive and that things won't be pretty.  When we see them again in Act 1, sc. 3, one witch explains how she is angry at the wife of a sailor who wouldn't share her chestnuts with the witch.  As a means of revenge, the witch is going to send winds to cause the woman's husband's ship to get blown off course and she is going to cause him to become thirsty and suffer from sleeplessness.  The importance of this is to show us how the witches, or weird sisters, work and what their powers are.  They cannot control human behavior directly or the witch would have made the woman share the chestnuts.  Apparentl,y since the witch, along with her two companion witches, sent winds to blow the sailor's ship off course, the witches can control the elements. They can also cause thirst and insomnia, it seems, since the witch claimed that was what would happen to the sailor and Macbeth suffers from insomnia after he kills Duncan.  After the witches share the scene with Macbeth and Banquo, the witches disappear while still being questioned by Banquo and Macbeth. This also shows more of their powers. Since the witches cannot directly control human behavior, they cannot make Macbeth commit murder and that is an important element for us to remember.  The tyranny of Macbeth cannot be blamed wholly on the witches.  They may have done some nurturing of the seeds of ambition, but they were already planted and Macbeth allowed those seeds to blossom.