This is a great question. Augustine's City of God is one of the most important books that was written in late antiquity. Also there are many reasons why it was historically important. So, I will have to be selective.
First, Augustine wrote this book in response to what had taken place in 410 AD. This date was akin to 911 for Americans. Rome, the eternal city, was sacked by Visigoths. This was a huge psychological blow for the Romans. Something unthinkable happened and in many ways their worldview was shaken.
Second, Augustine's book also showed that Christianity was intellectually mature. His work is a massive tome of incredible learning. This shows that the church had developed greatly. Moreover, many Roman authors whose works we have lost are only recorded in Augustine's City of God, such as Varro. For this reason, this book is also important.
Finally, Augustine's book also played in important theological role in that it gave hope to Christians that even if worldly cities of glory fall, the city of God will always persist. This theological point gave rise to the importance of heaven for Christians. It should also be noted that this work is still read and loved by many people even today.