I never journal, but I like to do it when I'm on a trip alone. Even though, I like to think that I'm never truly alone, whether it's because I rent a room in a private home, at an hostel, or taking part in an excursion. I always meet people.
I do it to keep a record of my trip and also because I have a lot more time to myself. Usually, I journal in the evening, in bed, to reflect on my day and keep a memory of my trip. I use a small notebook someone gave me or one that I bought and kept in my notebook box. A few years back, I started bookbinding, so I also make my own notebooks.
It's been a while though. I should consider taking a new solo trip !
Little tip: For those who like Midori Traveler's Notebook-style, it's super easy and way cheaper to make them. There is a bunch of tutorials on YouTube, like this one. I made a few ones with scraps of saddlery leather someone gave me. Well...someone gave it to someone who gave it to someone who gave it to me ;)