Here are three reasons why you need to keep a journal as a developer. Number one, it will keep track of your progress, whether you are learning a new technology or doing a new side project. Documenting your progress in a journal will increase the chances that you stay consistent and you finish what you start. It will also serve as a list of your achievements at work and help you negotiate that promotion more effectively.
Number two, keeping a journal will improve your writing and documentation skills. Writing every day will push you to get better and this will help you in your daily basis at work. Whether you are writing a ticket or any kind of writing you are doing, it will automatically improve. And finally, a journal will allow you to reflect and learn from your past mistakes. You will be able to look into the past and analyze, hey, what went well, what didn't, and what can you correct?
As a pro tip, if at the same time you want to improve your communication skills, you can also try keeping a video journal.