Why People Write Diaries?

I think the reason of why people write in a diary is because they want to remember what have they done or something interesting that happen. The only person I remember that was famous in diary is Anne Frank. She have written almost two to three years diary. Her diary also have become books to sale in the world. The reader will wants to read other person's diary is because other people can know what kind of life does he or she has. They also can know what does he or she have done before, or what interesting thing did he or she faced before.