In addition to what others have said about honesty and ownership of mistakes, it shows you how much they're capable of learning from the inevitable mistakes.

Even more importantly, if they haven't made any major mistakes yet, there's a good chance they'll make it once you hire them, because they haven't been through the whole experience and have no scars to show for it. Or maybe they haven't had yet a job where they were responsible for anything that could result in a major incident. There's no lesson in paranoia as great as wiping the wrong storage array, shutting down services in an order other than the sanctioned one, etc.