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.
i think that second meeting gives Macbeth an over confiedence which could be included in fate as well. it gives macbeth confiedence which leads to his downfall. that's all i can say.