The letter of Macbeth was important to Lady Macbeth. It reinforced for her that the two worked together as a couple. Her husband confided in her, and respected and depended upon her advice and guidance. He didn't just tell her that they'd have the honor of King Duncan I as their house guest at Inverness Castle. He also updated her on his interactions with the three witches and their predictions of ever greater things for him on the job. Because she was warned in advance of the visit and of the predictions, she could prepare her household for her guest and her victims.
