There's no question, the Camino journey is a personal one. Yes, you'll meet other pilgrims and you'll also meet many other people along the way in the refugios, hostels, hotels, restaurants, and shops. Still, the Camino is personal. You'll experience many different emotions. You'll also see and hear and taste many different things. Keeping a journal is a way to record these daily experiences for yourself.
Journal writing is also an excellent way to explore topics, thoughts, emotions, and ideas that come up while you walk (or bike or ride). There's something about being in unfamiliar places, about hear and seeing and smelling that conjures up thoughts and ideas and memories. Your journal is an excellent place to capture and explore.
While walking the Via de La Plata, I also found that my journal was the perfect place to collect passport stamps. I'd have my Pilgrim Passport stamped for verification of my journey, but I'd also have my journal stamped. This created a very nice memory that's easy to pull off the shelf and revisit.
Of course, having a bit of paper on your trip can also provide some practical uses, too. A place for notes, directions, and even paper to rip from the book to leave a message along the way.
My journal also proved an excellent place to keep track of lists (lists of cities and towns, names and emails, menu del dias that I particularly liked). It also helped me keep a few bits and pieces I'd picked up along the way handy and in order.
And, while there are many electronic options for keeping notes and images, I really like the feel of writing and the idea of having a tactile book of memories.
Of course, paper and books add weight to our packs. So, the choice is up to you what you want to carry. And, paper and rain don't go well together. But, I kept my journal inside a sealed plastic bag.