According to this hypothesis, each time a choice is possible, the universe is separated into several parallel universes, one for each possible choice. Then, the notion of choice is an illusion. Let us suppose for example that I can chose between A and B and that I chose A. In fact it is only in this world that I chose A. In another parallel universe, I or rather my duplicate has chosen B. I have the impression that only this world is real and the other is only hypothetical, but my duplicate has the impression that his world is real and my world is hypothetical. In fact in the absolute all worlds are as real.
One could then conceive that the tree structure of all possible worlds would be entirely predeterminated, and that we have an impression of free will, which would come from the fact that we perceive only one branch of this tree structure, and this branch is not predeterminated.
The many worlds hypothesis also permits to imagine that there exists an infinity of universes, one for each possible set of physical laws, giving an explanation to the anthropic principle.
Some people reject this explanation because they consider that resort to universes to which we cannot access, and which from our viewpoint does not really exist, complicates the things. But as we saw it previously, the only true reality is our consciousness, our perception and action at present time; all the remainder is mental construction; and this construction has to be the most simple which agrees with experience. Paradoxically, the many worlds theory is not more complicated but is simpler than the theory of the unique universe, if we look from the adequate viewpoint, if we consider the "germ" and not its unfolding : the "germ" of all possible universes is smaller than the "germ" of a particular universe like our universe. One formulation would be the phrase "For each possible mathematical model of a world, there exist a corresponding physical world." is probably shorter than a hypothetic book of physics describing the "theory of everything" of our world, theory that remains to be discovered and that could be infinite.