The Parson is a religious figure who's actually fulfilling his vows and obligations. He provides a point of comparison for religious figures like the Monk and Friar, who are not. As the Parson lives the obligatory life of poverty, the Monk and Friar are living the high life. As the Parson gives to the poor out of his own meager salary, the Friar takes the last penny from a poor widow. The Parson pays careful attention to the words of the gospel, applying them to his own life; the Monk disregards the words of religious authorities, declaring those texts worthless to him. And so it goes. These characters require each other to have their full effect.