There are all kinds of diaries, and some are more detailed than others. Shauna advises trying to "find personal accounts of areas where your families lived, and don't forget to look for military unit histories and diaries to supplement what you have found in army dossiers. If you have never thought of exploring these types of records before, why not try and find a shipboard diary for an ancestor’s voyage. You may be pleasantly surprised." Finding a diary of and ancestor's voyage out to Australia or of their military unit's war experiences can give you a weath of information, even if the diary if not written by YOUR family member. Reading the thoughts and reactions of another person experiencing the same thing can really make the history come alive.
Of course, finding the diaries of a family member is just a treasure! I am lucky enough to have some of both my father's diaries and my maternal grandmother's diaries, and some of their handwriting is a challenge to decipher. While I only have a few years worth for each of them, and neither exactly filled pages with the minutae of daily life, both are invaluable in filling in details of major family events and my own early life and I treasure them.
After my father's death my mother and I found some of his diaries hidden at the back of an old wardrobe, and I am so lucky to have them. My father had Alzheimers and spent several months before he died 'cleaning up'. Months later I was still realising things were just gone - including the entire contents of my family history filing cabinet! His diaries date from my childhood and are patchy at best - some have weeks and even months with nothing recorded - but several give me exact dates for major family events and his (often surprising) reactions to them.
My grandmother's diaries are similar - they date from my childhood and are patchy in coverage - but these were given to me by my grandmother, who knew I was interested in family history and knew I would find them interesting, and those few little books contain her view on family events I remember dimly at best.
So whether the diary is actually written by your ancestor or by someone on their ship, in their military unit, or living up the road in the same village, diaries can offer you a wealth of detail that may not be available anywhere else.
Visit Shaun's blog on Diaries to read her full entry on this topic.