Why Keep a Diary ?

Diary, according to a dictionary's definition, is a book of a person which keeps daily events and experiences. Hence, it could become a great memoir someday if I were trying to tell my grand children some stories when I was young. To me, a diary should be kept by a mighty person or a legend because he/she needs to give some evidence in order to prove something to other people. Therefore, I wonder if I kept one for myself, would I become a legend someday? ha...however, keeping a diary has still not become my habit because I don't want to be cautious all the time because people might unveil my personal experience or secret someday when I lose my diary.