Why keep a diary?

To me, to keep a diary is to keep my memories. When I was small, my mother used to force me to keep a diary in English so that I can practice how to write in English. However, because I really don't know what to write, I wrote "I am happy today." everyday. Now, when I look through what I wrote, I felt really funny and recalled my feelings when I was writing the "diary" When I was in middle school, I kept a habit of keeping a schedule book. Although it is not a diary, I almost wrote down what I did everyday. And now, when I flip through the books, I can recall what I did in that year, in that month, and at that day. I don't know what the purpose of writing a diary is when I was small, so writing a diary is a painful thing to me. Now, because I found fun looking through what I wrote, I know that the purpose of writing a diary is to keep my good memories fresh, and I found fun writing it.