What is Chaucer's main reason for writing about the pilgrimage in "The Prologue" from The Canterbury Tales?
Chaucer had a difficult time with the hypocrisy he so often saw in the Roman Catholic Church during the medieval period. Priests who were to be celibate, taking a vow of poverty, often kept women and lived better than their poor parishioners. Those who were supposed to help the unfortunate often ignored or took advantage of them. Pardoners would forgive the sins of those who could line the Pardoner's purse.
This seems to be Chaucer's purpose in writing The Canterbury Tales. The Prologue is Chaucer's way to introduce the members of the pilgrimage (a journey to a holy place in order to earn favor in God's sight and improve the condition of one's soul) to the reader. Chaucer not only pretends that he is one of the pilgrims, using this "tool" so that he can travel with them and observe them, but in life he was known for being a student of human nature. He is very observant and relays not only vivid details of the appearance of each pilgrim, but studies their behavior to point out those who are truly pious (holy minded) and those who are hypocrites. Many see this as a satirical writing.
Chaucer is very critical of those who pretend to serve the Holy Church while taking advantage of the poor. However, Chaucer does credit some of those in his traveling party who were truly decent folks. The Knight is one.
The Prologue's premise is that the travelers all agree to tell a story each night when they rest at the inn to entertain each other. In this way we meet each character, seeing him/her from Chaucer's point of view.