I've heard that the point isn't conversions, it's to put members in an intentionally uncomfortable place with "outsiders" who are probably going to be rude or at least dismissive. It makes everyone outside their group seem hostile, and then you come back to your community and everyone likes you and praises the hard work you're doing (even though it doesn't actually accomplish anything).

I have no idea if anyone actually put any thought in to it like that. But it certainly seems like a reason why it would be worth continuing.