Soliloquies show the characters' thoughts and feelings
1. A soliloquy is a long speech by one character that is not spoken to any other character on stage
2. It's just them thinking out loud about their emotions, so it's a good way of showing the audience how a character is honestly feeling.
Some important soliloquies in Macbeth:
Lady Macbeth's speech to the spirits in Act 1, Scene 5
Macbeth's speech about killing Duncan, in Act 1, Scene 7
Macbeth's "Is this a dagger which I see before me" speech in Act 2, Scene 1