These postulates are not theoretical dogmas but presuppositions having a necessarily practical reference and thus, although they do not indeed extend speculative cognition, they give objective reality to the ideas of speculative reason in general (by means of their reference to what is practical) and justify its holding concepts even the possibility of which it could not otherwise presume to affirm.
Theoretical reason, we learn in the first Critique, cannot inform us as to the existence of God, freedom, and immortality. However, following pure practical reason requires that we assume that these things are real. So we have reason to believe in them. (Freedom has a special role among the postulates of pure practical reason in that we can also non-sensorily detect it when we non- sensorily detect the moral law.) The reason why we must believe in the postulates is the linkage of practical reason and the good. Although action from practical reason is always motivated solely by dutifulness, it also aims always at the highest good. The highest good is the rewarding of the virtuous with happiness, so it looks like we cannot assume that acting dutifully will produce the highest good. If this were right, we could make no sense of acting morally. However, if we assume that there is an afterlife and that God will reward or punish us there in accordance with our goodness or badness, then acting dutifully can aim at the highest good, and morality is possible.