Kinda like a lawyer. You don’t care if your guy is guilty or innocent - you just want to win.
Kind of like a lot of professions. Most folks in professional sports probably don’t work for their favorite team, and they even play against them periodically, and yet they go all out. Advertisers don’t necessarily think the stuff they’re advertising is any good, but they put together the best campaign they can. The military doesn’t always believe our foreign policy is worth a damn, but they still defend our freedom to give them the wrong mission. Really when it comes to beliefs there’s two types of jobs: the ones where what you believe doesn’t matter, and the ones you shouldn’t let your beliefs impact your performance. Debate club is really useful for folks go on the second path.