He made me start a journal.
Yeah he kinda just said it would help to be able to get my thoughts down
Did it work?
— Mae Borowski, talking to either Bea or Gregg about her journal.
Mae's Journal is a journal prescribed to her by Dr. Hank after The Incident, as a way for her to cope with her mental issues. The journal is used by Mae throughout the game, and can be looked through by the player whenever. After the ending credits, the player can skim through Mae's Journal, to see how many pages they've obtained.
After the incident, the journal was given to Mae by her doctor, Dr. Hank, as he assumed this would help Mae cope with her anger management. Since then, Mae has used a journal to doodle in, as a way of summarizing her thoughts, as well as what she's done that day.
Despite being a main part of the game, the journal never directly affects the story and it's events. The journal acts as a summary of the story, and tells the player what Mae is thinking of during certain events.