The mulitiverse theory, basicly, is that everything that can happen, does happen. It doesn’t ‘decide’ to split if you change history, or if two snowflakes collide or don’t. The possibilities already exist.
Anyway, who’s to say we have free will or thought? For all we know we could be playing a role, like mindless zombies. Not a pleasant thought, but just as possible as anything else.
I guess the big question is why? If there are infinite universes, what would the point be of their existence? If time travel is possible, it would prevent paradoxes. Is that the only reason, like some sort of self preservation mechanism?