The most common way to use a journal is to record events, whether over a long period of time or for a shorter event like a trip. The main benefit is to help you remember the things that you did and the people that you met over the course of your life, especially the ones that were important to you. It’s also a good benchmark to see how your priorities shift. You might start out by talking about who you sat with on the bus (as I certainly did in some of my elementary school journal entries) and eventually note new jobs and routines in your later life.
Many studies have also shown that this type of journaling has other benefits, such as the improved memory that comes with recalling the events in order to record them in the first place. The catharsis of writing can also help reduce stress and anxiety.
This type of writing is so intimate that it can draw people in and capture the imagination. A person’s diary might be less structured than a three-act drama, but it also helps you get to know the subject better. That’s why there are so many diaries published, like The Diary of a Young Girl by Anne Frank, and so many fictional works portrayed as though they were partly or entirely written in diary format, like Bram Stoker’s Dracula or the Dear Dumb Diary series by Jim Benton. Try reading a book like this, then a book written in first person that’s not a journal, and finally a book written in third person. Compare the effect they all have on you.