From William Boyd's Bamboo (2005), a collection of his critical writing, this lovely riff on Keeping a Journal:
For the journal - relating as it does a life-story, or part of a life-story - does so in a manifestly different manner than the other forms available, whether biography, memoir or autobiography. All these last three are fashioned by the view backwards, informed by the 20/20 vision of hindsight. Only the journal truly reflects, in its dogged chronology, the day-by-day, week-by-week progress of a life. Events have not yet acquired their retrospective gloss and significance; meetings and people, projects and schemes have not matured or developed.
The impenetrable judgments of the future more often than not undermine the honest analysis of the present. That job you were so excited about has not yet turned tedious; that thriving dot-com company you sunk your savings into has still to go belly-up; that pretty woman/good-looking man you met at the party last night has not become your wife/husband - and so on. The journal has to have the same random shape as a human life: governed by chance and the haphazard, by that aggregate of good luck and bad luck that everybody receives. Biography and autobiography dilute this inexorable fact, shaped as they are by the wisdom of hindsight and the manipulations of ego, and are literary forms that are, in many ways, as artificial and contrived as fiction. But, by definition, a journal cannot do this: it's written as the future unspools into the present.