I don't really understand journaling

Before and during taking care of my dad while he had cancer, I started to journal. I am not 100% sure why, though, and that fact has been bugging me lately. It didn't really feel good or relieve any stress, I didn't share it with anyone (not like that would have helped anyway), and I still cannot bear to look back at it due to the sad feelings it reminds me of. Maybe one day I will look back at it, but I doubt it. I guess the point of this is that I do not fully understand the point of journaling or diaries. Like, do people write down the mundanities of their life and then just never look at it again? What benefit does that provide?

I could understand that it would be for documenting what was going on at the time, or like life lessons and teachings that were imparted at the time. But I guess it is a little late for me to try and document anything about my dad at this point. Too late to cry over spilled milk, though.