Sometimes during game play, it becomes useful to have a place to make notes to yourself.  Things that you want to keep private, whether it's secrets that you've uncovered during game play, or lines that you want to use, or things that you don't want to forget, etc.  This is a place for you to create notes that will only be seen by you.  Some other uses might include, posting character sheets or stats, spell lists, power lists, catch phrases, verbiage of arcane spells (so that you don't have to get the Latin translation or equivalent each time), and clues.