>>9*SPOILER FOR OTHERS*
Sorry for the necro, but I also don't exactly know why they were there. I think it had something to do with the Emperor. If he didn't eat them up, I guess he'd go eating the world. It left a lot unanswered. If they really wanted to, they could do a second season of it.
Hell, where Nakiami got sealed away for 1,000, they could do it where the seal broke away earlier than expected and the original characters can meet up. Won't happen, but it's left open.