In terms of Chaucer's The Canterbury Tales, and specifically his "The Wife of Bath's Tale," the central idea of this story relates generally to how the story was structured by the author.
Chaucer decided to build his story based on a structure called the "frame story." He uses a pilgrimage, a popular journey to holy places, as the frame around which to build his story. It is brilliant in that it was the one event which would join people from all socio-economic groups, including the simple, humble Knight; the successful and husband-seeking Wife of Bath; the poor and dedicated Parson; and, even the deceitful Pardoner.
This group of people is thrown together when they travel together on a trip to the shrine of Saint Thomas à Becket, who was murdered in Canterbury in 1170...
...by Henry II's soldiers—within the very church walls.
It would be rare that this variety of social classes would ever cross paths. On the journey, Chaucer (who—astoundingly—includeshimself as a member of the group, using his name) reports what he hears. And each night, the Host of the group (identified as HarryBailley) of the Tabard Inn, requests that each member of the group tells a tale to pass the time. Chaucer's Prologue introduces the members—vices and/or virtues included.
Each tale is supposed to "support" some aspect of the speaker's character. For the Wife of Bath, who is looking for a husband, she tells a tale that extols the virtue of a man marrying a woman regardless of her looks, whereby he may be rewarded in other more "passionate" ways. However, Chaucer is making sure to use these characters to make his points or point out a moral—or both.
If the knight in the Wife of Bath's tale were punished at the beginning, there would, first, be no tale to tell. However, more importantly Chaucer wants a chance to make his point and it is done through the Wife of Bath's "chosen" story.
In having Arthur allow his Queen and her ladies to judge the knight, the Wife's point is made of the ability of women to make important decisions. Perhaps the year the knight takes to find the answer to their question indicates how far removed he is from understanding a woman. The question put to him is central to Chaucer's tale.
But that the other ladies and the queen
So long prayed of the king to show him grace,
He granted life, at last, in the law’s place,
And gave him to the queen, as she should will,
Whether she’d save him, or his blood should spill.
The queen she thanked the king with all her might,
And after this, thus spoke she to the knight,
When she’d an opportunity, one day:
“You stand yet,” said she, “in such poor a way
That for your life you’ve no security.
I’ll grant you life if you can tell to me
What thing it is that women most desire. (37-48)
The knight is sent out to find out what every woman wants. In keeping with the story, the Queen and the women will spare the knight's life (which was forfeited by law in raping a woman) if he can tell them the answer, thereby learning what they believe is the most important concept a man can know: how to truly make a woman happy.
Chaucer, of course, has his fun with the answer: a woman is happy (as the knight eventually reports) when she gets her way in all things.
The function of the knight (from the Wife's point of view) is to show the foolish side of men, but indicates that they can be "taught." The knight creates the conflict whereby the tale may be told and Chaucer's point made.