My progress journal has served a lot of purposes for me. Firstly, I have at times used it a regular diary, where I can vent about whatever's going sideways in my life, it's not too often that I go there but there are moments where it's a necessity. Secondly, writing can be a very lonely business. I'm not fortunate enough at this time to have any close friends I can share my stuff with, so a progress journal is the best I can do right now. Even if it's mostly illusory, the feeling of being seen is valuable. Thirdly, there's an element of accountability. Whenever I have an active progress journal, there's a small voice inside me that occasionally checks in with "what news do you have this week? Would they be proud of yours progress?" And fourthly, the historical value. I like to be able to go back in time, see where my mind was at six months ago, a year ago, see what thing I was working on and struggling with then. Fifthly, sometimes I wrote down snippets of insight in my journal that hadn't really made it into my writing yet, and it's nice to have a reference base. Back in the OG Forum I wrote something in my journal about PPP (plot, people, places) which was a storytelling method I was toying with for a new project. It seems very basic when I talk about it like this, like yes obviously you're going to have those things, but I had a whole system for it. My journal is the only place I wrote the method down, and I'm kinda bummed now I didn't save any of my old progress journal. I still retain the method in my mind, but a bit of nuance was lost.
In short, we aren't all lucky enough to have a writing partner in our lives (I had that for a while but lost it) and it's important to have a space to casually talk about your stories. I try once and again to talk about my stuff in the real world, I have people willing to listen, but they aren't readers or writers of the kind of stuff I produce. I get a lot of polite nods, but that's it. Having a progress journal lets me engage with a community of like-minded freaks.