On further thought, I don't think it will give them evidence information (maybe timeline??), but maybe psychological information. Ok, so they were told to look stoic on tv, maybe they don't grieve openly, but in a private journal there should be some evidence of her grieving or lack there of. Maybe they are looking for lack of remorse.
I lost 3 children due to premature birth and I can tell you it takes months where you can even breath without your heartbreaking. It takes a while before your thoughts are not so consumed with them that you can think about something else and that would be evident in a journal. Of course she states she thinks Maddy is still alive, and that maybe almost worse to think about. I would be crippled by the thought of who my child might be with and what my child might be enduring. So to see where she is mentally, yes, the journal would be huge. If she is talking about how far she ran, who she met, etc in a private journal, that would be very telling.
But still VERY stupid to tell her in advance!!