In Act 1, sc. 1, the witches' familiars, Graymalkin and Paddock, call to the witches.  The familiars are in the form of a cat and a toad and without the influence of the witches, they would not be familiars.  In the third scene of Act 1, the first witch explains how a sailor's wife was mean to her, so her revenge was to cause the man to suffer insomnia (just as Macbeth suffers from insomnia after he kills Duncan) and to cause his ship to be blown off course.  Apparently, the witches can influence one's sleep and they can influence the winds.  Also in Act 1, sc. 3, the witches influence Macbeth with their predictions.  He and his wife decide, as an effect of what the witches said, to kill the king so that Macbeth can become king. In Act 4, sc. 1, Macbeth goes to the witches again.  They show him three visions initially, that influence him in two different ways.  The first vision says to beware of Macduff which influences Macbeth to kill Macduff's family.  The next two visions make Macbeth feel invulnerable to defeat.  The fourth and last vision that the witches show Macbeth is of a line of kings going from Banquo to the current king of England (King James I).  The influence this has on Macbeth is mostly to make him more desperate and paranoid.