You wrote that staging should be the same as production env (and I agree), so how does it help with catching new errors if they were already caught and reported

You wrote that staging should be the same as production env (and I agree), so how does it help with catching new errors if they were already caught and reported on production? Of course we could run that against our test environment, but then I'd be afraid of getting to many errors, that were caused by changes that we want to release. So is there something obvious that I'm missing?