It's the increased frequency (5 "story emails" in first month trump 1 "transactional (welcome. login here…) email") and the fact these emails are a chapter in a story, specifically that someone has registered for your service: you can assume that you have piqued someones curiosity with your proposition: now push (well, drip) it down their throat with a sequence.

(I would not have been so confident with my copywriting if i didn't know it.)