What was the purpose of the witch in The Chronicles of Narnia?

The stories that make up The Chronicles of Narnia are allegorical, that is to say they represent a hidden story or meaning. The author of The Chronicles, C.S. Lewis, was a devout Christian and intended his stories as Christian allegories that would reveal the message of the Gospels through an epic struggle between good (Aslan) and evil (Jadis).
To that end, the character of Jadis, the White Witch, represents Satan and all his evil powers. Satan is a tempter, constantly trying to get fallen man to depart from the path of righteousness and immerse himself in sin. In the New Testament Satan even tries to tempt Jesus himself, offering him all the kingdoms of the world if he would only fall down on his knees and worship the Prince of Darkness. The White Witch's offer isn't quite so generous; in The Lion, The Witch, and The Wardrobe she tempts Edmund with some Turkish Delight. But the principle's the same: Jadis, like Satan, attempts to divert humankind from its true path, seducing us into sin with fleeting worldly pleasures.