Diaries for some are treasured. It is the book for little secrets. The book where are the dirty stuff is written that no one is allowed to read. For some that may be the way they look at their diary. A diary does not have to be looked on as that. It can be a simple journal. A place to put your thoughts. Writing down what you did on a particular day, or writing down a particular life decision you made.
Writing down what happened in your life helps to recall your memories. It also tells your life's story, which may not be of so much interest to you as it is to your children. Stories about a family heritage are important. This information gives family something to look back on. Whatever you call the book it is a record of your life. You could put in the book what you had for breakfast; what you did that day, or any thoughts you had.
It also will help you to become a better writer. It could also help you with your phraseology and organize your thoughts. A diary or journal is for your life's story, and it is an important thing to put on paper for the most important reason, it is about you.