Oh shoot, so sorry for the very delayed response on this!
It's a bit confusing but there's 2 uses for the journal: one in-story that only shows up during select moments and then outside of the story in achievements that's accessed via the button. The one in-story operates by looking for you to input certain bolded keywords that show up in the scenes beforehand in order to get more "special" entries from Barbara compared to what you get if you enter anything else.
An example being: [SPOILER] "Clown" for Cameron!