I think it is a bit dangerous to put your thoughts on paper. This is why I don't write one. Anne wrote a diary, because she felt alone in the world, without any real close friend to talk to. She wrote all her feelings and secrets and impressions of life in it. She even named it.
People write a personal diary, because they feel free with the diary.
It is like a good friend. It only listens and never answer impatiently. We can feel very intimate with it. I don't write a diary, because I have great friends with who I can write about everything. A few years ago I kept a diary.