Writers of murder mysteries - and not all mysteries involve murder, but it's a solid example that lets us get our hand around it - usually need to reveal the culprit at some point. It's better if this revelation makes some kind of sense. So they have to know before the reader who exactly done it? I suspect that the best way to do this is to write the events leading up to the murder, then the murder itself. Then bury it until it's time to show everyone.
Amnesia seems to be the same thing. If a character can't remember something crucial, the author still has to know what it is. So there's a lot of writing you have to do, and then keep mum about.
Just sort of thinking out loud here.