This quote is very important in terms of the whole play, as it actually could be used to summarise a major theme of this excellent tragedy. The appearance of the witches and their speech points towards the paradoxical nature of what happens in this play and the shocking reversal of values that this points towards. Also, it points towards the confusing nature of what happens to Macbeth as he tries to work out how to respond to the prophecy that the witches give him and work out what his role in achieving it should be. You also might like to think of how this statement foreshadows various points in the play where what is actually "foul" is disguised as being "fair" and vice versa. Consider Lady Macbeth's advice to her husband in Act I scene 5 for an example of this:
...look like th'innocent flower,
But be the serpent under't.
This quote directly relates to the gap between appearance vs. reality, which is something that Lady Macbeth is able herself to achieve perfectly, as is demonstrated by the way in Act I scene 6 she is able to present herself as the perfect host to Duncan, whilst secretly plotting to kill him. The statement of the witches therefore points towards a setting where the normal values of the world have become dislodged and are replaced with an ever-shifting moral quagmire of seeming good and hidden evil.