Shakespeare's utilization of soliloquies in Macbeth is of extreme importance. The function of the soliloquies is to direct our attention to the characters' personality, their thoughts, feelings, motivation, etc. Only through soliloquies do we understand the inner world of the characters and their ambitions, aspirations, and fears. Without soliloquies in Macbeth, the characters, especially our tragic hero, Macbeth, would be robbed of their humanity -- they would be viewed as mere tools for plot development. But Shakespeare wanted more than that. He wanted us to understand his characters as complex beings, who are multidimensional, embodying both the good and the bad.
For instance, when we examine Macbeth's soliloquies, we gain insight into his inner world -- his thoughts, his ambitions, his fears. Without the soliloquies, we could easily cast him as a villain, whose uncontrolled hunger for power makes him cold-blooded and vicious. The soliloquies, however, help us understand his multi-layered character. The soliloquy about the invisible dagger, for example, helps us see how insecure and uncomfortable he feels about his plan to kill Duncan:
Art thou not, fatal vision, sensible
To feeling as to sight? or art thou but
A dagger of the mind, a false creation,
Proceeding from the heat-oppressed brain?
Another soliloquy which can even evoke in us feelings of sympathy for Macbeth is the one in which he states how futile life is:
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more: it is a tale
Told by an idiot, full of sound and fury,
This soliloquy, like many others, help us understand that Macbeth is not always ruthless, confident and unbowed. He is a human after all, and his failure to make his life meaningful can evoke a sense of sympathy among the readers despite all the atrocities he committed. The soliloquies define his humanity. And that is why they are important.