It's been most helpful to me for tracking work and health issues like I said. e.g. being able to say specifically "I wrote the software for A,B,C,D,... and also did E,F,G,H,..." in my performance review instead of forgetting half of it. Reflecting on how many days I'd listed health issues was useful for getting myself to seek professional help instead of just sleeping walking through life as well; still working on that, but having the evidence there I can say things quantitatively like ~25% of my entries (923 days out of 3623 logged) include "tired", "exhausted", or "unproductive"... Life isn't supposed to be like that.

I had a bit of trouble getting started initially, but forced myself to do it for a couple weeks until it became a habit. For me it's part of winding down towards sleep now; not doing it feels like not brushing my teeth.

It won't solve your problems for you, but it can help you see them if you have them, and having an auxiliary memory to brain dump into is also nice since you don't have to keep everything in your head. I recommend anyone who can get into the habit do it; I wish I'd started sooner.