It is important for Macbeth to realize that his mind is playing tricks on him before he kills Duncan because it highlights the internal conflict and turmoil he is experiencing. By recognizing that the dagger he sees before him is not real, Macbeth shows that he is still capable of rational thought and is not entirely consumed by his ambition and desire for power.
This moment also serves to emphasize the psychological complexity of Macbeth's character, and the extent to which his thoughts and emotions are driving his actions. It suggests that he is not simply an evil or malevolent character, but rather a flawed and vulnerable human being who is struggling with the weight of his own ambitions.
Moreover, Macbeth's recognition of the illusion of the dagger is a turning point in the play, marking the moment when he fully commits to his path of destruction and begins to act on his murderous impulses. It adds to the dramatic tension of the play and reinforces the tragic nature of Macbeth's fate, as he is ultimately undone by his own weaknesses and flaws.
Overall, Macbeth's realization that his mind is playing tricks on him is an important moment in the play, as it reveals much about his character and sets the stage for the tragic events that follow.