I've heard the benefits of a diary going two ways (sometimes both simultaneously):

- You realize that your life was following an arc that you didn't realize at the time. e.g. you worked on similar projects, dated the same type of person etc but you can only see that when you look back in retrospection.

- Your perceived arc was really a series of random events in your life that you made the best of.

Both of these can be true because lives are complex and multifaceted. e.g. your dating life can be on an actual arc but your career can seem like an arc in hindsight but your diary shows that you just took whatever job seemed to be most interesting at the time.