The first prophecies by the witches are that Macbeth (currently Thane of Glamis) will be Thane of Cawdor, and later king, and that Banquo's line will be kings, though he will not be. They are important basically because they set the plot in motion. Before meeting the witches, and hearing what they predict for his future, Macbeth is a loyal vassal of Duncan, and a noble warrior. After the encounter, his conscience and his sense of loyalty begin to give way to his ambition. There are still plenty of chances for him to turn back, and his wife is of course central in his decision to go through with Duncan's murder, but his descent into murderous tyranny began with the witches' first prophecies.