Diaries are a good way to preserve what you have felt at a time.
Diaries are a good way to pour out things you do not want or can not tell to anyone else.
But in plus, diaries are also good to get used to writing 'stream of consciousness' or in the flow, being in the moment.
My first diary dates from Christmas 1944: that is at least when I got it and begun writing in it: I was 10 and half years old. It is the one on the top in this picture, showing three of my thirty notebooks I have written till now.
There were times I wrote almost nothing, times I wrote day by day, times I had to hide the last one more carefully. I did not read them back, unless I had a crisis in my life. Then the "old wisdom" from previous ages helped me to go through!
It was not till I retired, I read all through, decided to put the text in computer. Then to translate from Hungarian (mostly the first) to English and French, then all to French and add context stories and publish them on a blog. I realised by that time they could be interesting but no editor would actively distribute it, even where (seen with other diaries) they did publish it.
I published Retroblog, one note at a day. Alas, in reverse order as are in blogs. Then, with great work, Journal de jeunesse, the firsts ones, in "normal reading order". After arriving in London, I translated the first diary and the stories around that time in English too and put them in a blog.
My diaries, the original ones are my most valued possessions. When I had to run away from my last husband who talked to me all through the night berating me to make me fly from our home (to bring his future wife in our apartment as fast I was away) in the small luggage I carried out the 5 in the morning, all my diaries were there, together with my official papers and my portable macintosh containing their texts.
I was right. After that I had to fight for everything I owed to take from my past apartment.
Well, that is down the drain now even if it still hurts, as my wise friend told me then "you are lucky you succeeded to loose him".
And I was wise to take my diaries with me. Somehow, they do represent "me" my life, my foolish thoughts and dreams but also the wise observations and analysis and sometimes so clear insights from so early one!
My life might not have gone like a quiet river and so much "interesting" is still buried in those many notebooks, but finally I am happy for all the directions it took me. All the lessons I learned. All the joys I experimented. All the courage I can still take from them.