I started my online journal-like writing back around in 2000, when I started posting random amusing rants about things on my now mostly-defunct website. Those are actually reasonably well-done essays, in the style of my random weird long rambling letters to people, only more focused.
I've been writing in a paper journal since January 16, 1991, the same day the Gulf War started. In the ten years between 1991 and 2001, I managed to fill up a ten gallon plastic tub worth of assorted paper journals: computer printouts, actual bound books, looseleaf binders, spiral-bound notebooks with my random braindumps. Now I put them online.
If it comes out on the page, it doesn't have to stay in my head. I like the ability to Google it, and it's easier to put these things here where a good 80% of the people I actually care about will read it (my father and assorted aunts included). Eh. It's a thing.