The reason has to do with her physical state and the need to wait before resuming marital relations with her husband. In this respect it is part of the overall pattern of laws of taharat hamishpachah, “family purity”.
In addition, one can read a theological significance into the immersion. It says that the woman has gone through a spiritual as well as a physical and human experience. With her husband, she has quite literally shared the Divine creative role.
Through creating new life, husband and wife feel in their own way a little of God’s ecstasy at the moment when the world came into being. As God must have said to Himself, “I am awestruck at the miracle I have performed!” so the new parents say, “What a little bundle of miracle our beautiful baby is!”
If anything can reinforce our belief in God, it is the privilege of childbirth and parenting.