I like to have a diary to carry in my handbag, and I use it purely for bring stuff relating to my diabetes (blood pressure readings etc) these days. As a younger person, I used to write in a diary all the time, and when alone or worried (I'm twice widowed) I did find that writing my thoughts down helped to sort them out, and worry less. It was a way of getting things "out of my head" by putting them on to paper. It is useful if you don't have someone close, in whom you can confide. When working, my diary was essential for appointments. These days, everything is written on to a wall calendar, kept in the hall, by the telephone table (and yes, there is still a telephone on it, even though hubby and I have personal mobile phones).