On a good day we may gain clarity of thought or at least expression, which is vital for communicating Buddhist insights to those who haven’t encountered them.
We may also identify and correct our own wrong views, if they are not too heavily armoured.
We may identify places where our belief is sincere but out of date.
We gain insights into how religious thinking and religion works, so that we are less susceptible to our own gullibility and credulity around religious ideas.
Being proven wrong is good for deflating the ego. And the only way to do this is allow someone else to analyse our views and point out the faults.
In the Enigma of Reason, Hugo Mercier and Dan Sperber, point out that, contrary to long held views, individual humans simply do not use reasoning to solve problems. Reasoning evolved to be a collective activity and logic only kicks in when we are critiquing someone else’s ideas. When presenting our own case, we constantly fall into confirmation bias and other informal fallacies. But when we listen to someone else’s presentation we are able to reason more clearly about them. So if we want to be rationale Buddhists, we must argue with each other!
Lastly, for all the conservative traditionalists out there, it is traditional for Buddhists to argue (viz the Kathavatthu), at least as far back as we have written evidence, so arguing helps connect us with our ancient roots and traditions. It is the duty of all Buddhists to preserve and participate in this ancient activity.
The Buddha was always arguing with Brahmins, Jains, Kings, his own followers, etc etc. It’s just that our texts present these as bloodless, passionless affairs, where no one ever puts up any concerted resistance to orthodoxy. Like he tells the Kālāmas not to listen to other people, and they just roll over and say “Yes, Lord, we totally accept everything you say without question or argument or even the slightest quibble” (the next bit of Brian’s speech from the balcony—“you all got to think for yourselves”—is never enacted in Buddhism).