For me, writing in a diary is a stress relief. My diary takes less work to express myself to me than writing something for other people to understand.
And, I listen to myself when I write to me. If I tell something to someone, they may not be listening or understanding. Writing in a diary could also be called free writing. It encourages creativity.
I think people should keep a diary. It is like having a conversation without talking to anyone. For me, the process and act of writing in a diary creates a sense of order in my thoughts, like it could for other people. It can certainly help anyone who needs to be a friend to themselves.
What I write in my diary is not what I would write in a journal.
A diary exposes deeper thoughts and it is easier to write out my thoughts. I think many writers create a journal instead of a diary. They are careful with what they write, believing that people would want to read their words one day or fearful that someone would read their words one day.
There are options. A writer can keep a journal and a diary. Let people read the journal and they will not look for a diary. Whichever way you go, the purpose of a diary and a journal is to communicate with yourself honestly.
If you are not writing a diary or journal, try it out. You certainly do not have to do it every day or even on a schedule (I don’t). You can write in a notebook or pieces of paper when you have the need and/or desire to talk to yourself.
If you are keeping a diary, that’s pretty good.