Penitence

The Pardoner's tale, in fact much of The Canterbury Tales, is about penitence. It's about a pilgrimage to seek penitence from a saint's relics. Penitence was a subject very important to Chaucer, and on the minds of all medieval Christians. The effect of sin was to separate a person from God, and in order to reestablish the connection and reconcile with God it was necessary to repent. In the early Middle Ages, subjective feelings of remorse seemed to be the essential ingredient for forgiveness of sin. But around 1200 AD, Pope Innocent II established a requirement that everyone had to go to a priest for confession once a year in order to be forgiven. Just being contrite wasn't enough.

From then on, the emphasis started to shift away from personal feelings of penitence and towards the necessity of the sacrament of confession. (Source). Personal remorse no longer was sufficient for absolution. And even if a subjective feeling of penitence wasn't totally there, the sacraments could still absolve sin. Forgiveness could only come through the sacrament of confession to a priest, or the purchase of pardons (also called "indulgences") authorized by the Church.

This was the context in which the Pardoner was practicing his trade and why he could make a killing promising absolution from all kinds of sin. He certainly wanted to elicit feelings of guilt and remorse in his audience, but not because that's what would lead to absolution; the people he preached to knew that wasn't sufficient. It's because it would lead to them opening their wallets to buy his pardons, which would do the trick. Today, the Church recognizes that a sincere wish to repent is an important part of the sacrament of confession (now called reconciliation, to reflect its purpose). But penitence was big business in the Middle Ages, which is why it's a major motif in the Canterbury Tales.