If you are referring to his constant referral to his inadequacy or his "don't get mad at me, I'm just reporting what I see and hear" comments, he does this to both protect himself from the present-day authorities (who did indeed have the power to throw him forever in a dark dungeon or put him to death) and also to put a little distance between the speaker as a character and the author of the piece and the other travelers along for the ride. In constantly reminding the reader that he is only there as a person on his way to pay homage to Thomas Becket. This gives him a little credibility since he has no connection, no loyalty, no one is paying him to portray the members of the church or the rich people in a positive light. In fact, the people he protrays in the most positive light are the Knight (a lower nobility), the plowman (a lowly farmer without whom we would not eat), and the parson (the plowman's brother who actually practices what he preaches). Chaucer's message to all of us is the same: practice what you preach (hey, Mr. Politician! Are you listening?), and to take care of those who are responsible for feeding the rest of us (aid the family farmer--don't cut his legs out from under him and add so many laws on the books that prevent him from being able to afford to make a living for his own family).