This is a great question. I would say that the Aeneid by Vergil (alternate spelling Virgil), does an amazing job when it comes to fulfilling its objective in providing the Romans with a national epic. Let me give you three reasons for this.
First, it ties Rome back to the beginning of things. In other words, Aeneas, the main character goes back to the heroic age of Homer. His journey to establish Rome goes back to the Trojan War, which was the most famous war in the ancient mind. In this way, the Romans can claim to come from the heroic age.
Second, the characteristics of Aeneas, particularly, his duty (pius) is one of the most important virtues among the Romans. Hence, by using this idea Vergil can impress upon the Romans the importance of duty to the gods, the people of Rome and family.
Third, the work was an instant hit among the Romans. So, the people judged it a worthy work for their national epic. This is probably the most important point. The fact that it has lasted this long should say something. Moreover, some of our founding fathers (of America) had Aeneas in mind when they established America.