I get asked why I keep a diary. Also, I forget why I go to the trouble.
Life goes by. Writing a diary forces me to think about the path I’m on. Not just the what but the how. How do I feel?
You will forget. I forget the day to day, but I also forget the big things. I forgot one of the best days of my life until I stumbled across a photo. Now I write it down, and index the big things.
The characters of Office explain the value of recording a life nicely (watch to the end for tears):
A diary is writing the story of your life. It keeps me present & focused on living it too. Time flies. Taking a few minutes each day to think about your life, to enjoy it as it happens, will compound in all kinds of ways.