In The Canterbury Tales, why would Chaucer add a retraction to his work?

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).