There are many novels that are written in diary format. Personally it annoys me, but I don’t like any kind of epistolary novel. Other people love them, so if that’s what you want to do, then go for it! ☜(ˆ▿ˆc)
It’s a good idea to keep a journal. Doesn’t have to be a journal specific for novels, though. There’s so much you can reap from your daily journals that could be useful for your fiction. Things like emotion, inner thoughts, environment and people observations and conversations.
I suppose the benefit of having a dedicated journal is that you can jot down specifics. I just use the notes app for quick thoughts and write it properly on my personal wiki.
I keep field journals for my research though. I use my observations to construct a narrative. It’s the same principle as keeping a journal for your novels/ working ideas.
The same way you would use a daily journal for what happens in your daily life and more, you would try that method for the novel you are writing.
Like, I would not only jot down notes and research materials for the story, but daily goals, my emotions towards the chapters I’ve written, word counts that I reached, my feelings on the story and things of that nature.