The soliloquies contribute to Macbeth's characterization as a tragic hero rather than a villain. Because we are able to know Macbeth's thoughts as verbalized in the soliloquies, we understand his dilemma and temptation as he contemplates killing Duncan. We realize that this murder is not easy for him and that he is fully conscious of the fact that this murder is wrong on many levels. But the temptation to be king is so great that it overrides his scruples.
As we see Macbeth succumb more and more to the forces of darkness, we see the accompanying sorrow and pain, and Macbeth's poignant realization of the horrible consequences of his actions. One of the more powerful soliloquies occurs in Act 5, when Macbeth feels that
My way of life
Is fall'n into the sear, the yellow leaf,
And that which should accompany old age,
As honor love, obedience, troops of friends,
I must not look to have . . .
Tragic heroes, according to Aristotle, in their tragic falls develop a heightened sense of self knowledge so that they understand the justness of their fate. It is through these soliloquies that we see Macbeth painfully aware of his losses and the fact that his existence now is meaningless. Through these expressions, pathos for Macbeth is developed.
Most of the soliloquies in Macbeth are significant because they develop the dramatic irony and characterization in the play. For example, in Macbeth's soliloquy regarding the dagger in Act 2 Scene 1, the viewer/reader understands that Macbeth does feel guilty about his plans to murder King Duncan. He cannot reveal this sense of guilt to Lady Macbeth because she wants him to be brave and "manly" in his efforts. But the reader learns that Macbeth is not quite as cold and calculating as he appears to other characters.
Similarly, at the very beginning of Act 3, Banquo's soliloquy states that he fears that Macbeth has done ill deeds to get his position as the King. However, Banquo cannot reveal his doubts to anyone else for fear of what Macbeth may do to him if he were to learn of Banquo's feelings. This develops dramatic irony (and suspense) as we watch Banquo go on to have a civil conversation with Macbeth about the upcoming banquet.
The soliloquies help in plot development,reveal character.They are the windows through which we get a glimpse of the character's inner conflict like that of Macbeth.The soliloquy in Act 1 scene vii reveals macbeth's ambition to become king, his desire to murder Duncan although his conscience pricks him that he has decided take his dagger against his own master.Later when he sees hallucination before the murder is due to the conscientious soul. When he pepares to murder Banquo because of insecurity his fear and insecurity is revealed and he spends sleepless nights.THese soliloquies not only help us to understand the character and know their inner struglle but also reveal their frustrations. They are also means of self revelation to the characters themselves.