The importance of this short scene lies in the way that the imagery and the host/guest motif allows Lady Macbeth to show that she is following her own advice that she has just given to her husband in the previous scene. Let us remember her words to him in Act I scene 4, where she counsels him to:
...look like th'innocent flower,
But be the serpent under't.
This of course relates to the key theme of appearance vs. reality, that is so ubiquitous in so many of Shakespeare's plays. Act I scene 5 shows that this is something Lady Macbeth herself is an expert in. Consider the way that even the presentation of the castle lures in Duncan whilst concealing something much more sinister:
This castle hath a pleasant seat; the air
Nimbly and sweetly recommends itself
Unto our gentle senses.
Both Banquo and Duncan comment upon the "pleasant seat" of the castle, which is of course a great example of irony, because something extremely unpleasant awaits Duncan there.
Likewise, Lady Macbeth, as hostess, presents a perfect image of a loyal and faithful hostess to her guest, protesting that Duncan does her and her husband a great honour by staying with them in their house:
All our service,
In every point twice done, and then done double,
Were poor and single business, to contend
Against those honours deep and broad, wherewith
Your Majestly loads our house...
Everything conspires to make Duncan believe that there is nothing wrong and nothing being plotted, whereas all the time, we know as an audience that Lady Macbeth and her husband are plotting to kill Duncan in their own home. The dramatic irony heightens our appreciation of this scene.