1 – It will keep you honest and you can compare then to now.
2 – They highlight what you do too much and not enough of.
3 – What looks good on paper may not work in reality, as such going over old diaries (food, training etc) can help you see what worked, what didn’t and what you have yet to try because it’s never been done by you.
4 – One can give you a sense of direction.
5 – You can have anime characters or your childhood heroes on it, what other reason could you need for wanting one now.
All in all writing down what you’ve done or what you will do can help massively in training, nutrition and habit change.
While at the start it is a pain in the ass, eventually it becomes a part of your life, a good habit to have.
Many successful people in the world keep one, therefore if it’s good enough for them then it’s good enough for us, right 🙂