I could have lucid dreams without journaling, but a main difference that I see is that by journaling I give more attention to what are the dream signs, plus I see that the memories of dream vanish significantly after two days. By journaling I can revisit my dreams and the dream signs and so going to bed with the intention to recognize the dream signs or investigate the reality looking for dream signs.

For example: in my dreams it looks like I change clothes accordingly with the activity I do. What I didn't do is to actually change my clothes. If I can't remember when I changed my clothes, it must be a dream.
Other example: I see often myself during the dreams. But I can't see myself in the reality, other than in front of a mirror (that is a chapter on itself and constitutes both a very useful and very terrifying way to reality check), so it must be a dream.
But that I realized only journaling. Journaling increases your awareness.