During the February 23, 2008 broadcast of The Writer's Almanac radio show hosted by Garrison Keillor, Keillor told his audience about Samuel Pepys (pronounced "Peeps"), a diarist from London who lived from 1633-1703. I found the brief history lesson about Pepys to be fascinating.
Pepys made a New Years resolution to start keeping a diary, and started on January 1, 1660. He recorded all sorts of details about his life, including his infidelity. But he also wrote about things like the coronation of Charles II in 1660, the great plague of 1665, and the great fire in London in 1666. Only one newspaper existed in London at the time, and it was controlled by the government, so English historians have relied heavily on Pepys' diaries to understand what was going on in England at the time.
Pepys never intended for his diaries to be read by others, so he used bits and pieces of shorthand, Latin, Greek, Spanish, French, German, and his own secret codes. He stopped writing his diaries in 1669, and it was more than a hundred years before his codes were cracked and his diaries were published. There's probably an inherent lesson in this for anybody who thinks he or she is writing something that nobody will ever read.
Given the fact that Pepys didn't write for an audience, I find it ironic that his personal writings were used to piece together the history of England during that time period. He probably wrote for the same reason that so many other diarists write--to be able to point to something tangible that proves they lived. Life goes by in such a blur and it's easy to forget yesterday, and last year, and even complete decades.
I feel the need to keep a journal for that very reason, but I don't write in it often enough. Hearing this story about Samuel Pepys might be a good motivator.