This serves a purpose that is not addressed in the GTD system - recording what you have done and thoughts/notes that may occur to you as you go through your day.
My DailyAgenda is where I "log" my events and tasks, so it seemed like the perfect place to also capture what I'm actually working on along with any notes I may want to capture. But, as I think I've mentioned in the past I'm a bit lazy, so opening Obsidian finding the right place to log what I'm doing (as well as making sure I maintain formatting that I setup) takes time, so I found something that I like to help make this easier.
I've been using doing for a while now and I really like it - in fact, with how it is incorporated into my daily agenda, it is super easy for me to reference notes from days gone by and facilitates s smooth review at the end of the week.
For more about bullet journaling, see Bullet Journaling.