It's quite obvious that Madara doesn't have eyes and he's waiting for Zetsu to fetch his Rinnegan from Obito, but with Minato and Kakashi in the way, Black Zetsu opted to momentarily possess Obito to fend them off, and there is also the matter of the other Rinnegan Eye that could possibly be stashed away inside Obito's Kamui Dimension (the same way his supply of Uchiha Sharingans are stored there)
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There's also the matter of Obito's death and final act of redemption, the Kamui Dimension itself was modeled after Japanese Graveyards:
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So I can't imagine Obito leaving a corpse behind in the battlefield that Kakashi can bury afterwards, more likely that he's going to die inside the Kamui Dimension locking Black Zetsu and the Rinnegan with his corpse forever, foiling Madara's plan one last time.