He was sort of a retcon. Originally the Bijuu all said Naruto was the "old man's..." with the looks and such it was clearly going to be reincarnation/successor, but that would've completely ****ed over the Yin-Yang theme and Naruto and Sasuke being equals; Sasuke would've been irrelevant. As such he chose another route; Naruto and Sasuke being transmigration of his sons, and Hashirama and Madara being the same. This was a way to explain why Madara was the only one to ever gain the Rinnegan, and to meet Hagoromo and get a power up. After that Hamura was used as a parallel for Sasuke as Kaguya saw Hagoromo in Naruto (which can be retcon'd to how the Bijuu initially saw Naruto) and Sasuke as Hamura. He was also a way (with Kaguya) to make the Hyuuga relevant.