As I mull your question, it occurs to me that journaling serves different functions. So the thought thread continues, you've identified some of those functions. One of the functions of journaling is something like an adage in writing that writing is thinking - that writing helps clarify and sharpens one's thinking, including awareness of feeling states. So journaling for even a bit can allow me to catch up to myself - help me identify what's going on with me. Especially handwriting, which for me is somewhat laborious compared to keyboarding, requires me to center and listen for, discern what is more substantial in me (thoughts and feelings) from among all my habitual chatter. Since I can just vomit on the page when I'm keyboarding, because I am a fairly fast typer, keyboarding doesn't natively impose the same discipline of winnowing my thoughts and feelings .
Knowing that I can burn the scrap soon afterwards frees me at some level to be real, to face unpleasant truths, to put down things that I can't make sense of or that aren't coherent, but which I have a charge on amidst the welter of thoughts and feelings that prompt me to pick up the pen at that moment.
Other types of journaling are valuable to hang onto. I may not be ready to let go of something. I was also reading about the journaling technology described in the journaling section of this website, and there are ways to really work what one has written down. I refer you to those intro threads by Andrew if you're curious to learn more about those.