The half-a-book scheme preceded the invention of eARCs. It used to be called Webscriptions, and was targeted both at bargain-hunters - you got all of Baen's offerings of the month for a low bundle price - and earlybirds - two serial installments would come three and two months before official Book Day, with the final proofed version, in all formats, available a couple of weeks before stores had it.

The eARC was a later development, when it was found that there was a substantial market for getting the entire book NOW, never mind if it has typos or costs more. NOW!