Because often it is safer than telling someone else. Think about times when you were angry. I also am able to forget about it, not re-hash the incident or conversation repeatedly in my head. Keeping a journal is silly. So you say.
Some folks record nothing more than daily activities, the weather, who was born, and who died.
My journal is more of an spiritual journey of my soul. I keep uplifting and favorite birthday cards, or letters that touch me. I record favorite quotes, words to a song that speak to me, virtually anything that makes me think about the world around me. I record my reflections, my inspirations, my down moments, and my grand moments. In the beginning, writing in a journal saved me.