I've kept journals off and on since I was about 18. They have sometimes kept me more or less sane during troubled times -- especially therapy. I wrote and wrote when I was seeing t#3, trying to make sense of what she was doing. I don't think I've ever thrown any away. Sometimes I have looked back at them. The last time I saw a therapist, I looked back at some of the journals I kept when I was seeing t#3 and saw that I was remembering some things not quite correctly.
From the other end: My mother kept journals off and on through her life. When I cleaned out her house when she had to move into a nursing home, I kept them -- but told myself I wouldn't read them until at least ten years after she died. That time has passed, but I have only looked at parts of a couple, which were from before I was born. I probably will look at more off and on through the years. At this point, they are in part historical artifacts. But they also in part confirm some of my impressions of her.