What are the benefits of keeping a journal/diary?

Recording your thoughts and events that have occurred throughout the day in a book seems quite appealing to me. Any time afterwards, I would be able to look back and reflect and think, oh, so thats how I felt when I ate that super burger after a jog, or whatnot, or more personal matters.

However, if I were to write about personal matters, and someone were to read it, most likely terrible things would happen.

Do you keep a journal/diary? What benefits does it bring for you? What terrible things does it bring, or might bring?