Example:

"Hark! she speaks. I will set down what comes from her, to satisfy my remembrance the more strongly.
Out, damned spot! out, I say! One; two; why, then 'tis time to do 't. Hell is murky! Fie, my lord, fie! a soldier, and afeard? What need we fear who knows it, when none can call our power to account? Yet who would have thought the old man to have had so much blood in him?
Do you mark that?


Explanation:

Shakespeare uses a blank verse here because Lady Macbeth has gone insane. She has become delusional and does not really know what she is doing. The blank verse adds to the fantasy that Lady Macbeth is in, or the guilt that she so posses from helping Macbeth kill Duncan.