The letter gives the reader an opportunity to get to know Lady Macbeth, her inner thoughts, what are ambitions are, what motivates her. In this scene, we learn what Lady Macbeth thinks of her husband.
"It Is too full o' the milk of human kindness
To catch the nearest way; thou wouldst be
great, Art not without ambition, but without
The illness should attend it; what thou wouldst highly," (Shakespeare)
She tells us that he has ambition, but that she believes that he is too soft, too kind, not tough enough to go after what he wants.
She, on the other hand, is very tough, but is stuck in a woman's body and therefore must rely on her husband to take the lead and pursue her ambitious goal, to become king and queen of Scotland. She tells us that she will do her best to convince him to take the proper action needed at the opportune moment.
"That I may pour my spirits In thine ear,
And chastise with the valour of my tongue
All that impedes thee from the golden round,
Which fate and metaphysical aid doth seem To have thee crown'd withal" (Shakespeare)
Lady Macbeth is so excited about the possibility of her husband becoming king that she goes outside to implore, to beseech the dark spirits to assist her, to fill her with power, she begs the universe to turn her from woman to man for this moment so that she can take the action needed, killing the king.
"Under my battlements. Come, you spirits
That tend on mortal thoughts! unsex me here,
And fill me from the crown to the toe top full
Of direst cruelty; make thick my blood,
Stop up the access and passage to remorse,
That no compunctious visitings of nature
Shake my fell purpose, nor keep peace between
The effect and it! Come to my woman's breasts,
And take my milk for gall, you murdering" (Shakespeare)
Lady Macbeth advises her husband on how to act in the presence of the king, so that he can manipulate the situation to his advantage, she tries to teach him how to be deceitful.
"Your face, my thane, is as a book where men
May read strange matters. To beguile the time,
Look like the time; bear welcome in your eye,
Your hand, your tongue: look like the innocent