What is the purpose of the discussion to convince Charles to take Madame Bovary to the opera in Rouen?What is the significance of the discussion?

The discussion between Homais and Bournisien over the morality of the theatre represents Flaubert's attempt to display the battle over traditional and modern notions of the good.  The pharmacist, the supposed epitome of rationality advocates the opera.  He may believe this from the enlightened stance he possesses, but he might also be advocating this because these are elements that represent wealth and the growing emergence of the middle and upper classes.  Going to the opera is seen, in this light, as a sign of social mobility.  The priest is representative of the traditional view, that such experiences like the opera is a moral distraction from the true notion of spiritual worship.  He might believe this form of the spiritual stance, but he might also be advocating this because the priest is aware that as material wealth is increasing, spiritual fervor is on the decline, resulting a lesser base of power for people like himself.  Flaubert does not take a side in this debate.  Observe his language in the text, as reporting the debate in a "realist" manner.  The other reason he doesn't take sides in this is because Flaubert is going to end up calling both sides as wrong, for both really seek to create a more conformed view of the individual.  Notice that both of them are fighting over what Charles should do.  Charles is a tool between both that pharmacist and the priest.  Flaubert understands that neither totalizing vision can cure the pain that lies at the heart of nothingness, and understands the propensity for abuse in each singular vision.  The discussion between both religion and rationality is one that is present within the mindset of the modern individual, the audience for which Flaubert is writing.  He shows both sides, if nothing else, to depict them and show how both will not assist human beings.  Charles decides to take Emma to the the opera, but nothing good comes out of it.  At the same time, Charles pursues a faint hope that taking her to the opera will help her health.  This means that like the modern individual, Charles uses his freedom and is disappointed by it.