These journals have proven very valuable to me as I've worked on projects and needed to reference information. I started by recording information that my leaders would relate in meetings. They weren't necessarily things they were saying directly to me, but items that would come up in planning discussions with the entire event management team, things about event management that I just didn't know at the time. It could be anything--ratios to use when renting certain equipment (at least 1 portable toilet per 75 people and 1 ADA portable toilet per 150 people); standard revenue split percentages for merchandise sales with partners; keeping signage messages and PA announcements positive (use REMEMBER TO rather than DON'T FORGET TO); and dozens of other simple things.
From there, I started summarizing great articles I read (a lot of early Fast Company articles in my first journal), writing down key points from books, and taking notes from a multitude of speakers I heard. It's progressed now to the point of clipping key items that I know I'll use later, especially data/statistics and their sources. (Even though I know it could all be found relatively easily again on the internet, I still clip it and tape it into my journal. I know what's in the journal and can flip to it even faster than I can Google the exact info.) The latest evolution of what I keep in my professional journal includes ideas that I someday want to turn into my own business(es), sketches, names of people from my network I'd want to hire and in what roles, what I want my company employee policies to be, etc.
I have a friend who negotiates contracts with professional athletes for his organization. Last month, he was telling me that his mentor had kept meticulous notes of all his negotiations throughout his career. My friend started doing the same when he began working for his mentor. He said that it's his professional journal to help him with future negotiations, but it's also a great way to document his career for his own edification.
If you decide to keep a professional journal, make it something that is your own and fits your personality. There's no one way to do it. The only rule is to use it in whatever way is going to be beneficial to you.