The mitzvos are the medium through which a person can create or destroy himself. G-d must judge the person through an existential assessment to see what he has acquired, and if this person has somehow created the seeds for eternal existence or the seeds of annihilation. The World to Come is a place of perfection; there is only existence or non-existence. G-d must judge and purify souls so make them homogenous, so that they either merit to be in the World to Come or merit annihilation.