Author Anne Spencer Morrow (wife of Charles Lindbergh) put her finger on a fundamental truth when she observed, "Writing is more than living, as it is being conscious of living." Even (maybe especially) the young diarist knows how the blank page can be a wonderful way to "work things out." No audience is necessary, or even desired. The privacy, the very anonymity of it all, invites thought on fire. Some continue the therapeutic practice throughout their adult years.
Compare such private stream-of-consciousness writing to the demands of compelling fiction as reflected in, say, novelist Ernest Hemingway's articulated machismo, "There is nothing to writing. All you do is sit down at a typewriter and bleed." Of course he bled. Not for nothing was he the esteemed Hemingway.