The Talmud states that a person learns the entire Torah while in the womb, but when he is about to be born, an angel appears and strikes him on the mouth to make him forget it (Nidda 30b). Why does a person need to learn the entire Torah if he will subsequently forget it? It is impossible to understand God’s Torah without divine assistance. However, if everyone remembered the Torah he had learned in the womb, he would not be able to receive reward for Torah study. Therefore, he forgets the entire Torah before birth and reacquires it only through hard work.

A person is incapable of understanding the Torah without divine assistance:

This Torah is concealed from the eyes of all living creatures, and humankind in this material world has no possibility to grasp even a tiny iota of it without divine assistance…

The reason that one forgets all the Torah he learned while in the womb:

Behold, if [understanding the Torah] is [solely the result of] an act of the eternal God, there will be no possibility for reward and punishment, since it all will necessarily be decreed by the Creator. Therefore, one is made to forget all the Torah when he comes into the world so that he will subsequently acquire knowledge of the Torah by his own will, through effort, and by choice, with complete awareness.

The benefit of studying Torah in the womb even though it is subsequently forgotten:

The act of learning, the fact that one is taught all of the Torah in his mother’s womb, is beneficial, as something of the divine assistance will remain etched within him such that he, with his earthly abilities, will be able to retrieve it…. This is the desired outcome of teaching a person all of the Torah before he comes into the world and then causing him to forget it: So that the matter will come to him through effort and by means of his choice. It is unattainable without divine assistance, but since it remains in his memory, he is able to retrieve it through his labor and effort. (Rabbi Tzvi Elimelekh Shapira, Benei Yisaskhar, Sivan 1:5)