In Macbeth, what is the purpose of Act I, Scene vi?

This scene serves two main
purposes:

1. to reinforce the basic
   good-heartedness of Duncan's nature, showing that there is no reason that he should not
   continue to rule as a wise and just King,


2. and to begin
   to build the suspense that leads up to the murder of Duncan.  To create this suspense,
   Shakespeare uses dramatic irony.

Duncan,
immediately upon entering, says, "This castle hath a pleasant seat."  A double whammy of
dramatic irony (In scene v, the audience has just heard Lady Macbeth say that Duncan
"never shall" see the "morrow.") and support of the open-hearted way that Duncan is
prepared to embrace Macbeth and his wife.

Banquo also adds
to the irony when he says, "Heaven's breath/Smells wooingly here."  The irony here, of
course, is that Heaven must be far from this place, as Lady Macbeth has just called upon
"thick night" to "pall [her] in the dunnest smoke of
Hell."

The balance of the scene is an interaction between
Duncan and Lady Macbeth, which adds to the dramatic irony and suspense, but also reveals
how very cunning and cool Lady Macbeth can be.  She doesn't bat an eye as she lies in
the very face of the man heaping compliments upon her, knowing that she will be witness
to his death before the next sun rises.

For more on this
scene, please follow the links below.