Michael stated that he started writing because he was depressed. Journaling is also a technique used by tons of counselors from Church teaching to Cognitive Behavior techniques.

Here, i think that it's a great tool to remember things that happen or happened while under stress. Information is the key. It's a great way to gather information from different perspectives and also a way to double check your own reality.
Memory is faulty, but when you have it written down and you need to recall something or bump it against some new information, you have it at your finger tips. No need to try rely on your faulty gray matter.

The last thing is that it's a damn good way to be remembered in case you end up taking a dirt nap or turned.