Why is paradox important in "Macbeth"?
It has to be explained within the contemporary context of the play.
When Shakespeare wrote Macbeth, two aspects of the contemporary culture affected the plot.
Firstly at that time there was a belief in a hierarchy within society where everything had its place. This in turn reflected the medieval feudalism of the time.
Secondly although we associate Shakespeare with Tudor times, Macbeth was written after James VI of Scotland became James 1st of England. As such he was the first Stuart monarch and Shakespeare re-wrote Scottish history to reflect this fact.
Given these factors the play is about a character who seeks to overthrow the natural balance of society. Therefore it is full of paradoxes in order to explain why this is unnatural; and the natural scheme of things must be re-instated.
As a consequence, from the very opening line, "fair is foul and foul is fair", which is not only a paradox, but is also linked to Macbeth when he says, "so fair and foul a day", the central theme of the play is established. Macbeth is thinking about change which will upset the pre-ordained and necessary hierarchy of society. Inevitably this will ultimately fail and order and balance will be restored.
There are other paradoxes which reflect this. Despite gaining the throne, Macbeth's guilt is suspected immediately and thus his fate is sealed. Lady Macbeth seems to have the strength of character which her husband lacks, yet it is she whose character disintegrates into madness delusion and death.
Macbeth is assured by the witches that no man born of woman can kill him, but Macduff was from his mothers' womb untimely plucked.
The paradox also lies in Shakespeare's conscious misinterpretation of Scottish history. In reality Duncan was not a good king, Macbeth was. Indeed Macbeth was secure enough to undertake a pilgrimage to Rome. However Duncan's descendants were the Stuarts and Shakespeare could not write a play highly critical of his monarch's forefathers. So, even here there is a paradox.