Very much. But my useful is mostly about personal stuff and once in a while revisit. Looking at things in retrospect. Sometimes I get something to learn, sometimes it's just fun or once in a while it makes me sad too.
I have been doing it on notebooks (paper journals; and not the fancy and famous ultra costly ones). Really. It's very recently that I started to put some entries on Android Journey app which are kinda too personal to leave in my daily diary which I don't lock in a box. So, I will recommend a paper notebook and pen. I am from India and I am always in the hunt for good and affordable notebooks and fountain pens - we don't have many in the former category. Anyone looking for something good here try Brahma Series (or Brahma Forces) notebooks - they sell Moleskine like notebooks (honestly paper quality is actually a lot better) minus Moleskine's marketing cost in its price. (disclaimer: nothing against Moleskine)
Not really. Some days I just write about something that happened. Or something I thought or just anything. Some days I skip. Somedays I just pen a poem or so (or something that I kinda thing are poems). A short story. Or a sketch (yeah, I make sure the notebook I get have good pages and works well with pencils, ballpoint pen and fountain pens - the latter being my favourite), a caricature, a schedule, sometimes card game score when I am travelling with my friends and we had a game, travel itinerary. I have found some pages where there's some math of splitting trip expenses among friends. Something from a book I am reading, or a film I watched, or something about it. I travel/backpack a lot and that's when my diary gets maximum attention. But these things don't happen very frequently and it's mostly the typical diary entries. So, it's my personal diary and I wrote it just because I like it. It it helps me in my development in some way then it's entirely coincidental and it wasn't the intention at all.
Yes, as explained above. It can just be anything.
Well, I don't think you could get something useful from my kind of journaling (I believe you are looking for journaling that is done from the productivity point of view and similar expectation; I maybe wrong though). Keep writing is all I would say. I have been writing since I was in class 6 - pretty much 20 years now. Yeah, frequency has gone down a lot since school. No doubt about it.