It is for book readers, because they just told you who is leading the White Walkers. In the books there has been absolutely no mention of who they are, who leads them or what they're doing to Craster's sons. The Night's Knight has been mentioned in a few legends and tales as A Very Naughty Boy but as a myth, legend or fable. I assume there's a big twist in the books coming to say "Yes, it is I! The man from your stories! I LIVE!" or what have you.
Shouldn't mean a thing to show viewers though, no.