1. Writing clarifies your thoughts.
The act of writing down your thoughts helps you clarify the way you think and believe. I once read that easy reading is dang hard writing (expletive removed). The reason for this is that a good author prunes his or her sentences until they're perfect. They are finished only when the group of words form exactly what they were thinking, and occasionally that group of words form their thoughts.
We are to always "be prepared to give an answer to everyone who asks you to give the reason for the hope that you have." (1 Pet. 3:15). Writing helps you prepare that answer.
2. Writing reminds you of how big God is.
This is especially true for people that keep a prayer journal. We're very forgetful of God's track record. He answers prayers we forgot we even asked. Keeping a journal or a calendar of when and what we prayed and when and how God answered those prayers helps us realize how much God truly cares about us.