It's actually very helpful! Even just to keep track of what you did and how you were feeling on a given day.

I didn't keep one for a long time because I grew up in a household where I didn't feel secure - if I had a journal my parents would have snooped for sure. As I've gotten older I (a) see the value in remembering the past (b) feel more trust in the people around me.

Highly recommend trying a journalling habit for a month. It's also very satisfying to do it in a little notebook with a pen instead of typing. It feels more tangible.