Why is Lady Macbeth's soliloquy in Act 1, Scene 5 important to the play?
This speech sets the mood for the horrible events which will follow...namely the murder of Duncan, which leads to the murders and deaths of so many others.
It prepares the audience for what is to come, teaches them about Lady Macbeth's character and what she is capable of, and also informs the audience as to the type of person Macbeth is. We know, for instance, from her speech, that he would not come up with the idea of murdering Duncan on his own and he certainly would not go through with this plan if she were not there to give him "courage".
The speech also sets up the theme of gender roles--Lady Macbeth at the beginning is more of the pants-wearing character by her own character analysis than her husband who is, according to her, "too full of the milk of human kindness" to do anything against his beloved King.
Setting these two up as strong vs. weak at the beginning makes for interesting comparisons later in the play when Lady Macbeth becomes weaker and more human...guilt-ridden and suicidal and when Macbeth begins planning murders without the help of his horrid wife.
Without that speech, the play would be a very different being. It is essential to not only the plot but character development.
check Approved by eNotes Editorial