you kinda answered the question yourself... you dont know when your gonna enter the dream, so you do the RC to catch yourself in the dream- when you fail the RC, that means your not in a dream and then you do the process over. Put it simply~ you fall asleep but you keep a part of your brain on and then your in the dream but since you believe your in your room, your dreamscape forms your bedroom and thats why you RC- to catch yourself