I was wondering why a keeping a dream diary is a good thing to do when trying to become lucid?
It helps you get on the habit of remembering more dreams. Which after time will help you to become lucid.
It increases your dream recall so you can remember more of your dreams. Then that’s supposed to get you lucid in your dreams.
It also enables you to actually remember a lucid dream when you happen to have one.
I would say it does three main things. 1. improves dream recall with daily use so you can remember your dreams. 2. It helps you find your dreamsigns 3.It prepares you for the world of Lucid Dreaming
What everone else has said. I usually have really bad recall but just after a couple weeks of doing a dream journal I had started to remember multiple dreams every night. It really works you should try it.
Keeping a DJ really helps improve you dream recall, as you devote more time to your dreams and all the details in them. The more you think about what you dreamt of, the more you remember plus its so much fun going back into your old dreams and reading them again 
Ive beein keeping a DJ for 6 years and i think it’s really worth it.