Leo,

For some people it is not easy to simply imbed in the subconscious that when you dream, you will be aware you are dreaming. Dreams can be fast-paced and highly visual and because that thought is not visual, it may not serve as a reminder once the person falls asleep. For more learned lucid dreamers, they may have discovered ways to properly plant suggestive seeds in their mind to achieve lucidity, but for a lot of neophyte lucid dreamers, visual is the first step. Once they inject a visual clue into their subconscious, it may be likely to show up and trigger a lucid dream. Once they become experienced with that, they can move on to non-visual reminders, such as simply "I will know I'm dreaming. Because of already having experience in their Dreamworld of achieving lucidity, it will be much easier for a non-visual reminder to trigger a lucid dream.

I agree with what you say, but not as a beginner's step to lucid dreaming. Simple mental reminders didn't work for me at the beginning, and may not for many others as well.