When I was young, I would keep physical diaries. Now I've kept online journals for over 10 years.
I've had those that relate to my life and thoughts specifically & those that relate to my health and weight. Now I try to combine all of the above for the mostpart, since they are all relevant.
It helps me sort through my own thoughts, catalog the events and feelings throughout my life, and tends to provide a great deal of motivation. If I am able to write about myself and the faults I may find in myself, I am further able to flesh out the sources of the fault or delve into other aspects for a wider spectrum or a positive outlook. I'm also able to hold myself accountable for myself if I've written it down. It escapes my mind, where I can just forget or ignore and not feel a drive toward betterment or awareness.
It keeps me mentally, emotionally, and physically healthy. Especially when I feel I can't directly share all of my thoughts to someone, based on how it may be too much at once or even just based on not feeling comfortable enough to do so.
Course, diaries and journals are completely dependent on how the writer utilizes them. You can write and get nothing from it if you aren't in tune with it, or if you don't know what you could draw from it.