I think Chaucer had multiple purposes in writing the Canterbury Tales, some of which we will never know! He clearly was disenchanted with the Catholic church and uses a lot of satire in his tales, definitely. One way many authors "got away" with criticizing the church was through satire, which involves poking fun at serious subjects. Chaucer is no exception. I do not believe that he had very specific purposes but that he wanted to merely bring attention to some of the inadequacies and wrongs in the church.