The reason for this rule/suggestion is because writing every day keeps your mind in the piece of fiction. Even when you're not working on a piece of fiction per se, your unconscious mind will be thinking about your fiction if it's something that you're working on every day. I know I've found this to be true in my own experience. Monday night I was coming home from a friend's house, and a great idea about a novel I'm currently working on occurred to me, seemingly out of nowhere. I don't think this would have happened if I wasn't working on my novel on daily basis.