I am quite influenced by the books I read. Books are quite powerful, and can provide a lot of information. When I was reading the book: Midwinter Turns To Spring, I was inspired to start writing journals. But I did not have a clear idea as to how I should make it useful for me.
We can write a lot of different things, and it all depends on the motive and the end goal. If you need a place to discuss about your own self, and put your self-talk into words, then you can write a journal. But I would recommend that it should be kept out of reach of everyone else, and be private to you only.
It depends on what you want to achieve by writing journals.