What is the importance of the city of Veii in the nonfiction book, The Early History of Rome: Books I-V?

The city of Veii was located near early Rome during the time of Rome's origins. It was of the Etruscan culture and located in Etruria. As a result of interactions with Romulus, there was a treaty between Rome and Veii that lasted for some time.

Near the end of the book, the Romans lay siege to Veii again, and later have a union with it. They nearly abandon Rome and migrate to Veii but decide to stay in Rome after all.