What is the dramatic purpose of Macbeth's decision to return to the witches in Act III, scene 4?
When Macbeth returns to the witches, he returns as something wicked. Even to the witches he is now in this category. In part it's showing how far he has fallen since they lay in wait for him at the beginning of the play. They are accepting him as one of their own - ie the devil's - kind.
Then, of course, the prophecies revealed at this time shape his behaviour for the rest of the play. His fear of Macduff is confirmed, and his mistaken belief that he is invincible stems from them as well. His fury at seeing Banquo's issue stretching out to the crack of doom as future kings of Scotland, showing that Macbeth's family will not hold the throne, probably makes him more reckless.
And of course, Shakespeare uses the occasion to pay a not-so-subtle compliment to the new King of England and Scotland, King James, who claimed descent from Banquo. The last King to appear in the line Macbeth sees in the mirror is thought to be James.