I don't have a diary, but I had two in the past. Once, in fourth grade, my teacher told me personally to keep a journal and give it to her every other week or so to show her my improvement of writing. My writing skill went up like four points in a year which is extremely good. Writing and science are both my top points in my all-round school career (my mother kept the records and you add all the points up to get your overall point).
My second time was when I kept a journal to record all thirty days of my first solo trip to New York to visit my grandparents. I just put down some jokes I learned, new things I learned that day, what I ate, what I did, etc. It was actually pretty cool when I read all the stuff a year later. Shows what your personality was.