I use Day One as well, on my laptop. It's a good app.

My journal entries are about things I have discovered or learned about myself in session, and things that occur to me between sessions that either builds on what I learned in session or is something new I might bring to the next one. Sometimes they're also about my feelings, particularly my maternal transference for my therapist at university, and recently I have been writing a lot of "letters" to her that I would never send but need to get out of my system somehow.

I also occasionally write a journal entry if I notice significant progress in an area of my life, just in general, relating it back to what I've learned in therapy to make this progress possible. I then also write an entry if I experience a setback.

Keeping a therapy journal has been a great help to me, as writing it down stops me from forgetting the things I've learned, and allows me to reflect more around it and possibly delve further into it on my own. It can be a great therapy tool, but whether or not it would be beneficial is entirely individual I suppose.