dreams are merely to keep the flow of existence. I think your brain constantly needs to see something happening. so since you're eyes are closed your brain switches over to "automatic/sleep" mode. that way other parts of your brain can take over and do the best it can to piece together a reality on past memories and how you think things will happen because of certain actions. you subconsciously know this.
So when you Lucid dream, those bits of information are just flowing in a more coherent manner so you would be able to act how you would in any waking day.
A dream reality is just the same as a waking reality
our genetics just suck so our bodies can't take 24/7 physical stimulation, or we might die.