By the time Macbeth has his second encounter with the witches in act 4, scene 1, he is in a much more desperate state than during his first encounter. In act 1, everything was going swimmingly in Macbeth's world. He was the courageous and loyal hero who had just stopped the rebel Macdonwald from usurping the throne. Everyone is impressed with his valor and Duncan makes him thane of Cawdor. When the witches prophesy that he will become king, he has the choice to take it or leave it. He doesn't need the witches: his world is quite fine.

By act 4, scene 1, Macbeth's world, however, is collapsing and his back is up against the wall. Being king, to put it mildly, is not what he expected. It has caused him nothing but stress, grief, and pain, and now Malcolm is marching into Scotland with an army to take the throne. His own troops are deserting him in scores, and he needs help badly.

Ironically, this time when he goes to the witches to seek information, they have been ordered by Hecate, the head witch, to play around with him and deceive him so as to lead him to his doom. Their intentions weren't malevolent the first time he met with them, but now they are. They speak to him in misleading riddles with prophecies that sound hopeful, such as that he can't be killed by a man born of woman, but they hold secret "zingers" that will be his undoing.

At this point, because he has nowhere else to turn, Macbeth puts his full faith in these prophecies. He's got nothing else. He has, as the saying goes, gone from hero to zero. He was a great warrior and follower of Duncan, but he has been a terrible, treacherous king, and nobody (for good reason) trusts him. However, although the prophecies are all he has, they are meant to betray him and they do.

The second meeting with the weird sisters in Act IV further shows the fracturing of Macbeth's mind; this was a respected, honorable and highly praised warrior, who has now been reduced to consulting with "witches" to plan his day. This is the man who defeated Macdonwald handily, and then took on the traitor Cawdor, defeating him and forcing the surrender of the King of Norway? Macbeth is scraping the bottom of the barrel - truly a man who is suffering from a splintering mind and soul. The question is, why? Macbeth the tragic hero continues tripping along, blinded by his fatal flaw (ambition) and a victim of fate (weird sisters).

This second meeting is important for three reasons:

First, the witches herald Macbeth's coming with "By the pricking of my thumb, something wicked this way comes."  For the first time in the play, Macbeth is recognized outwardly as a wicked person.  Before now, suspicions are raised, but no one has outwardly declared him "wicked".  There is a change in his personality and demeanor.

Second, that change in his personality becomes vividly clear when instead of asking the witches for information as in the first meeting, he now demands that they tell him what's up.  He would not have done this earlier, and it shows that he is taking more into his own hands.  Before, if you'll remember, Lady Macbeth did all the planning.

Third, the witches carry out their plan to set Macbeth up in false security.  They show him the apparitions which can be interpreted in more than one way.  They know, however, that he will see them in the way which plays out best for him.  For instance, "No harm will come to Macbeth until Birnam Wood marches up to Dunsinane Castle" is interpreted by Macbeth as the trees will unroot themselves and walk up the hill before you fall.  Impossible, right?  He does not consider the illusion of the forest marching.

In addition, the most important prophesy: "No one born of woman may harm Macbeth" is interpreted as no man can hurt him.  He does not entertain the idea of c-section birth, as Macduff was delivered.

How crucial is the second meeting of Macbeth with the witches?

Macbeth’s first meeting with the witches sets him up for his “triumph” of becoming the king of Scotland. It gives him the impetus to take action, murdering Duncan, as well as Banquo. The prophecy was specific to the outcome, but says nothing about the method. Macbeth decides he must himself take charge of the method, not trusting to fate to lead him to his destiny unaided.

The second meeting, however, is misleading. Each prophecy is interpreted by Macbeth as a guarantee that he cannot fail, when in fact it foretells his destruction. Macbeth cannot be killed by anyone born of woman. Macbeth does not understand the witches’ definition of “born” as a natural birth. Macduff was born, but it was not a natural birth but a caesarean section. He will never be vanquished until Great Birnam Wood marches to Dunsinane Hill. Macbeth takes this literally and completely ignores the word “until.” When Macduff’s forces use tree branches to hide their movements, it seems that the woods are marching. The second prophecy was meant as a warning that Macbeth completely missed. Instead, it made him overconfident and led to his death.