Yes I agree about staged rollouts, ability to 'unrelease' features, and so on. But let's say you're part way through implemeting a new feature, and you need to

Yes I agree about staged rollouts, ability to 'unrelease' features, and so on. But let's say you're part way through implemeting a new feature, and you need to alter a db table. How do you know that won't affect existing code (apart from tests)? How do you put that behind a feature flag anyway? As I said, deploying a change like this, without it being used for anything, seems like all risk and no gain.