What is the dramatic effect of Macbeth Act 3 Scene 4?Could you tell me what the dramatic irony is of this scene and what is the purpose of this scene?

When acted well, this scene is the highlight of the play.  We know that Banquo's ghost is sitting in the chair reserved for Macbeth, but only Macbeth and the audience see the ghost--thus, we have dramatic irony.  The scene is eerily humorous. Macbeth cannot control his reaction upon seeing the ghost and even shakes off Lady Macbeth's chiding that he is acting cowardly:

The times has been

That when the brains were out, the man would die,

And there an end; but now they rise again

With twenty mortal murders on their crowns,

And push us from our stools.

When he attempts to pull himself together and lament Banquo's absence at the banquet, the ghost appears again.  The audience sees the ghost before Macbeth does, and the anticipation of his reaction is enormous.  Shakespeare does not fail to deliver.  Macbeth is clearly shaken:

Avaunt! and quit my sight!  Let the earth hide thee!

Thy bones are marrowless, thy blood is cold . . .

This scene shows us the enormous changes that Macbeth has undergone.  With Duncan's murder, Macbeth instantly felt remorse and guilt.  He knew he would never sleep well again and wished that the murder could be undone.  However, with Banquo's murder, Macbeth's guilt becomes more subconscious.  He has suppressed his scruples over murdering his friend Banquo, even jovially praising the murderers who killed him.   Yet, we see that Macbeth is not an entirely evil man.  He is still wracked by his conscience, albeit suppressed.  The ghost is Shakespeare's dramatic way of showing Macbeth's inability to murder without suffering pangs of conscience.

It is after this scene that Macbeth completely distances himself from Lady Macbeth and turns toward the witches for help.