not sure. it doesnt say, might be to use them as
dna to revive the uchiha clan. or that they all
have different abilities. It might even be the
catch of him helping Itatchi with killing of his
clan, that he wanted eyes for it.
i think the rinnegan tobby transplanted to nagato
were the real madara's eyes. i think tobby
was there together with zetsu watching the fight
between madara and harashima, and the moment
madara died and harashima left maybe he took the
eye and then gave it to nagato. i totally agree
with hirokuroza about the sharingans tobby had.
and i think it wont evolve into rinnegan since it
has no body/host to put the eyes.