How does Macbeth being easily manipulated affect the story?

Macbeth is insecure about his "manhood" and what he wants in life. Initially Macbeth resolves to let chance crown him if chance got him Thane of Cawdor. L. Macbeth, however, has other plans. She wants to be queen and knows exactly how to manipulate her husband. She questions his manhood and gets him to do his bidding. The witches also know that suggesting things he wants to hear manipulates his what he wants and how he might choose to get it.