I spent two weeks this past November traveling through Japan with Sam Abell, a retired National Geographic photographer and mentor of mine. Sam believes strongly in keeping a photographic diary. He uses all the same care and skill capturing images for his diary as for his assignments. He believes that there is great power in documenting your daily life and those around you. As an extension of writers keeping a written journal, it makes sense for photographers to keep a photographic journal.
In order to do this topic justice, I wrote a somewhat longer article that you can find on my Composition area of this site. Here is the full article – Personal Documentary Photography – What and Why.