Mab is grooming Harry to be a new Merlin that restarts the white council.

[Bad Alias]

@Griffyn612: Margaret is the obvious answer to who had the reason and why. It's not necessarily the answer. It could have been intervention by someone else. Drakul states that the White Council is behind Harry's birth.

Margaret was called LaFey because of the amount of time she spent in the Nevernever and her facility with predicting ways. It also appears the Erlking knew her.

[Mira]

I'm not sure where Leah comes into things. Obviously she was tasked with watching over Harry, and was acquainted with Leah, but I don't know if Leah was involved in the Starborn plot, or aware but not involved, or clued in after Maggie went on the run.

I'd say that she was involved, I think it is very unusual for one of the Fae, let alone second in command to Mab, would involve herself at all with mortals unless it was vital that the child survive.

[Griffyn612]

Margaret was called LaFey and Lea was her only Sidhe contact mentioned. Lea knows intimate things about how she sees life and how that changed. From every small thing I read I get the impression they were pretty close.

I'm not saying they weren't, I'm just not sure how to define their relationship. Maggie seems to have known everyone, and everyone seems to have known her thoughts on matters. Kind of like Harry.  But I would hesitate to call him friends with everyone that knows him.

I'm interested in learning more at some point.