im going to keep it brief, so feel free to add to this theory
tobi is nobody. He is the creation of Madara using the rinnegan, just as the tailed beasts and moon were creations by the sot6p
his purpose was to return madara to life, whilst embodying his ideals and setting the plans of war into motions. Within his lifetime madara could not complete his task of becoming the next sage, and so using rinnegan izanagi he created tobi.
I wasnt sure if the other thread matched to this, seeing that was more of a split theory and sharingan izanagi than this
i just realised my first post said "Tobi is nobody" i called it! Haha