I think that this is a complex question. Different interpretations dominate such an analysis. I tend to think that the crown is so important to Lady Macbeth because of the power and title that it carries. She recognizes that such a title is something that is worthy of her husband and she wishes to do whatever she can to ensure that such an external distinction is conferred upon her husband. Lady Macbeth also understands that the prophecy of the witches combined with her own husband's reticence compels her to be the source of strength that can drive her husband. Lady Macbeth recognizes that her own coveting of the crown is to help her husband carry out with his own function what his sense of surmise is preventing. She knows that if she does not pursue the end of obtaining the title of King with so much fervor, her husband will not follow through. She recognizes that her passion can help to counteract her husband's own sense of weakness and lack of internal commitment to doing what she believes is a part of his own destiny and rightful embrace.