As he and his friend Banquo survey the battlefield, Macbeth says that he has never seen "so foul and fair a day." This, of course, is essentially repeating the line the witches sang as they departed in the first scene: "What's fair is foul/What's foul is fair." Shakespeare uses the witches' rhyme to establish a central theme to the play, namely that not everything is as it seems. Because Macbeth repeats the line in such short order, it can also put the idea in the minds of the audience that the witches have some power, either over Macbeth or to predict the future, that will be put to ill effect later in the play. What seems fair in the play is often corrupt and foul. Macbeth's rise to the throne, for example, is foul inasmuch as he murdered Duncan to achieve it. On the other hand, the witches' prophecies, which seem "fair" to Macbeth, actually bring about his ruin. His belief that he is impervious to death by all "of woman born" makes him overconfident, contributing to his ultimate demise. This is further foreshadowed later in the third scene, after Macbeth has been hailed by the witches as the future king of Scotland. Banquo asks him why he seems to "fear/Things that do sound so fair?" So by repeating this device, Shakespeare establishes the idea that appearances will likely be deceiving.
Macbeth, the tragic hero in William Shakespeare's Macbeth, first speaks in act one, scene three. His first line, "So foul and fair a day I have not seen," repeats the same paradox created by the witches in scene one of the same act: "Fair is foul, and foul is fair."
Through this, Shakespeare illuminates two distinct ideas. First, the phrase proves that something which seems to be one thing can, most certainly, be something very different. Secondly, Shakespeare is illustrating the parallel readers will find between Macbeth and the witches.
Historically, witches have been stereotyped as evil and dark. Through creating a parallel between Macbeth and the witches, Shakespeare foreshadows Macbeth's evil to come. Ironically, Macbeth, by the end of the play, proves to be the one many find to be far more evil than the witches.
Therefore, when speaking of rhetorical/poetic devices, Shakespeare links the use of the paradox to foreshadowing. The repetition of the phrase simply highlights the idea that some things are not always as they seem (highlighted by the fact that Duncan believes Macbeth to be an upstanding man).