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The soliloquy can be said to serve a couple of purposes. First, in Shakespeare's theatre, a soliloquy was really a device used by an important character in a play for conversing with the audience. In the Renaissance, there was no such thing as the theatrical "fourth wall" (an imaginary division between actors and audience which requires the actors to act as if the audience is not there). This was an invention of the 19th century. So, Macbeth is taking this opportunity to check back in with the audience, to fill them in on what he's thinking and planning. The first purpose of this (or any) soliloquy is to provide an opportunity for the actor to directly address the audience.
Second, the soliloquy exposes the character's thoughts and plans, as this speech by Macbeth in Act III, scene i does. The first line pretty much sums up Macbeth's frame of mind -- "To be thus is nothing/But to be safely thus." He can't enjoy being king until he feels safe, until he feels that there are no usurpers lingering around. And, as the soliloquy develops, Macbeth shares his plan with the audience to murder Banquo.
At this point, Macbeth seems to think that killing Banquo will solve his problem of being "safely thus." He says:
. . .There is none but he
Whose being I do fear.
He goes on to say that Banquo's valor and accomplishments cast himself (Macbeth) into Banquo's shadow, pointing out the ways that Banquo has, in Macbeth's mind, tried to push Macbeth aside in order to grab the glory for himself. This reasoning is important for the audience to see. They can see how Macbeth is becoming driven by his ambition and envy of others. His concluding words also display the greed that is developing in him, the reckless desire to win at all costs. He says:
. . .If't be so,
For Banquo's issue have I filed my mind;
For them the gracious Duncan have I murder'd.
. . .To make them kings, the seed of Banquo kings!
And with this, the murderers, whom Macbeth has hired to kill Banquo and his son Fleance, enter the scene.
This soliloquy shows the audience how Macbeth has moved away from the reasoning of his Act I, scene vii soliloquy, in which he rightly listed all the reasons it was a bad idea to kill Duncan. Now he is listing reasons that it is a good idea to kill Banquo and Fleance, revealing that, at this midpoint in the play, Macbeth is evolving towards his own downfall.