With the rapid advances of computers and the web, we often forget that computers only became wide spread a few decades ago. Before that, people actually wrote everything down and journaling was a key part of that.
Obvioulsy, the journals of some are more noted than others. Anne Frank, of course, is at or near the top of the list. She kept a journal of her life as she and her jewish family hid out from the Nazis in Amsterdam during the second world war.
The important thing about Anne Frank and all journals is perspective. While reading her journals is an emotional event, it is important to think of her perspective. She is not writing for you, but for herself! The journals are a story, but also an effort to cope.
It goes without saying that nobody should have to go through what Anne Frank and her family did. That being said, writing down your thoughts every day or two has some massive benefits and is something you should consider doing.
How sharp is your memory? Do you really remember the details of your life three, five or 10 years ago? Probably not. Journaling gives you an opportunity to create a timeline of your life that can take you back to events long lost in your memory.
Our society is all about doing things right now. This intensity can lead to a loss of perspective. Journaling requires you to take some time to reflect. As you sit and write, that reflection can give you a moment to be calm and make a good decision.
To this end, keeping a diary is also beneficial because it can help you see the big picture. At this moment, some aspect of your life may seem a calamity. Reading through old entries, reminds you that you've survived other situaitons and will this one.
Finally, keeping a journal gives you a unique opportunity to share your life with younger family members. You might prefer to keep it private, but the point is you have the choice to one day share your life experiences with a son or daughter that will amaze.